A sharing of my experience with firmware 2.06sg, 2.07TM, and 2.08TM:

Before I begin, I would like to say that the results below was solely my own experience. It may be vary for you guys. And note that I'm only using mobile phone (redmi note 3 SD), and this was the only device that connected to the router.

My 850L router came originally with 2.07TM and from what I've remember, it didn't gave me major issue with the wifi connection (2.4ghz + 5ghz) as well as the internet.

Then last week I've updated to 2.08TM, where the nightmare begun. The wifi connection on 5ghz disconnected certain times and unable to connect back with a message says password invalid. It can only be fixed by restarting the router. As for the internet, frequent delay or lag occurred. For example, if you would scroll Instagram or YouTube or even Clash Royale, it took as least 5 sec to load. Almost every time when i did a page refresh in the Instagram or YouTube, again it took about another 5 sec to load. I've tried to use the speedtest.net app to do some speed tesing, it couldn't even load up the "nearest location". Somehow the download speed was ok (full speed), but the whole connection thing went so delay/lag. And the 3rd issue was the 3rd party DNS (google + opendns) isn't allowed. Even though the 3rd party DNS is set, so-called illegal sites are still blocked.

Then I reverted back to 2.07TM and i thought the issues would be solved. Sadly the internet and the wifi connection were as bad as in 2.08TM. The only thing was working is that I could use the 3rd party DNS.

Currently the best solution for me now is to roll back to 2.06sg version. Everything goes fine so far with this firmware. The only thing is I've disabled the 5ghz wifi and solely use 2.4ghz, because disconnections still occur. I'm not sure whether it's the router itself unable to handle 5ghz or my device issue.

I've tried 2.07 global version, but failed. Probably it's due to the lack of vlan tagging feature in the global version. (Correct me if I'm wrong)

Conclusion:
2.08TM:
Wifi (5ghz) frequent disconnection, internet connection delay/lag, 3rd party DNS disallowed.

2.07TM (after roll back from 2.08TM):
Wifi (5ghz) frequent disconnection, internet connection delay/lag

2.06sg:
Everything is perfect, except for the Wifi (5ghz) has frequent disconnection

I hope my sharing here, perhaps, might help the community here.