It will happens occasionally. And when it does, the buffering icon and/or buffering usually would appear on-screen for 1-2 seconds.
Whether would you see this as a major annoyance or not, I'll leave it to you to decide.
Since this happens "once in a blue moon", therefore I don't see Astro is taking this matter very seriously, and rather put the blame on your WiFi or Internet connection instead.
Hmm... My Astro account is under Satellite, and I can interchange the box operating mode between Satellite and Plug & Play by performing Restore Factory Settings and still able to access to all my subscribed channels.

Since you opted for Ultra Box Plug & Play upon sign up, it means your Astro account is tagged under Plug & Play in their system.
Yes, you can Restore Factory Settings and change it to Satellite mode anytime you want, but still, in their system, your Astro box/account is tagged under Plug & Play.
Due to this, I'm not sure would this be the cause of you experiencing "Subscription Required" message on certain basic channels as you mentioned earlier whenever your box is on Satellite mode, which in their system, you aren't supposed to.

already once sent a technician to come to my house to check, but still, problems not solved.
Can share to us, what did the technician did when he came to your house? Replace new box? What else? Did he charge you?

If I'm not mistaken, there would be a 3 or 6 months installation warranty.
So if the same issue persists within the warranty period, I suggest you don't give up and keep chasing the customer service about this.

Anyway last week, I called the CS again and they opt me to change to new smart card (sim card)
Seriously I don't think it's the SIM card issue. 
But as long if they don't mention to you about contract extension or whatever nonsense one-time fees, then let them do whatever they want to do la.

Is it possible to switch the Plug & Play tag to Satellite in their system?
I believe it is possible. It'll be subject to their higher management's approval, I guess.
If you intend to switch your box operating mode back and forth between Plug & Play and Satellite modes, then you shouldn't have opted for Plug & Play upon sign up in the first place. 

Or need to change the box if switch?
No need.

I suspect because it's too tedious and probably involves lots of "red tapes" on their end to change from Plug & Play to Satellite in their system, so they'd rather take the easy way by telling you "No, you cannot change."

If there's anything you don't understand (especially technical things), you can ask here. Don't rely everything on their sales agents, cuz their priority is to earn your commission and achieve KPI, and not necessarily always advise you the best options possible. Not saying ALL of them la, but the way I see it, most of the salesman I see out there, are like that. It's a normal thing, I guess....

Because your account is tagged under Plug & Play on their system, and now you said you're getting "Subscription Required" message on certain channels when you switch to Satellite mode yourself, then I think they might sideline your case, saying that you are not supposed to be on Satellite anyway.

Aiyaa I tell you what... Try file an MCMC complaint that Astro refuse to change from Plug & Play to Satellite in their system, then I believe they'll take action.

I remember there's this case where a dispute customer publishes his story publicly onto Astro Facebook page, together with MCMC report on it. 

Similar story like yours, he thought Astro Plug & Play is an "amazing new product" but didn't dive too deep to know more about it. Upon sign up Astro, he wasn't informed that there will be streaming latency on Plug & Play of around 30 seconds to 1 minute compared to satellite broadcast. And their general T&C also didn't mention anything about it. And that's where drama begins.

He said that he's a live sports fan, and he is super pissed to see when his neighbours or friends around the world who watching the live sports via satellite, the football team scored the goal, while he only get to see the team scored the goal 30 sec to 1 minute later because he's on Plug & Play, which is an absolute killjoy to him.

I heard that eventually he got the higher management approval to convert this customer from Plug & Play to Satellite, without paying penalty whatsoever. And he agreed to pay some one-time installation fee to install the Astro satellite dish at his house, if I'm not mistaken.

So I think, maybe you can try something like that too.

Noted on that bro. I will try lodging a report to MCMC and see their consequent action. Thanks again for your kind reply.

So what did you mention in the MCMC report? You mentioned about getting "Subscription Required" message, or streaming latency, or what?
And when they change your account to Satellite, did they say something about arrange technician to come install Astro dish whatsoever?

Astro installer came yesterday to replace PVR with Ultra Box but did not change the LNB Universal type to DCSS type. He informed that he was not supplied the DCSS to change at the satellite dish. Upon installation, my TV could receive the UHD transmission with HDR at channel 805 and the installer maintained that there is no need to change the LNB. He asked me to contact Astro CS if I still want to change to the DCSS LNB.

I then called CS and was informed that Ultra Box and 4KTV which is HDCP 2.2 compliant are only required to receive UHD transmission. I told her that would be the requirements for Plug and Play installation but as my case involved the satellite dish why no LNB change but she replied not necessary for the LNB change as my TV was already receiving the 805 transmission.

However later on in the day when reading the Ultra Box Installation Guide (the installer left it inside the carton box during his installation) I found out on Page 17 on Satellite Antenna Connection where the DCSS is specifically mentioned.

So why did the installer and CS staff still maintain no need to change LNB to DCSS? I give the benefit to the installer that he was not provided with the DCSS as it was not difficult for him to get to the satellite dish at my house as it was installed at the balcony area on the first floor and he could just sit there to change the LNB.

I have since written to wecare@Astro on this matter.

Like to check whether any of you have recently changed to the Ultra Box using satellite connection and was the the LNB changed ?

Also what are the problems if the LNB is not changed to DCSS type using the Ultra Box ?