Like for any other telco, the advice is to get a prepaid SIM and test it out at the location(s) you frequent.

There's no general difference in network performance between prepaid/postpaid plans but individual plans or add-ons may be capped as the telco sees fit (they will say so if capped).

Also, I wanna remind you that VoLTE will not work on iPhone, meaning you can't make/receive any phone calls if you use Yes 4G on iPhone (except those Internet-based calls like WhatsApp call).
Mobile data will work as usual.

I strongly advise you to make a report to Digi and tell them the issue you're experiencing. When it's not on any speed cap, could you share on what are the:
- speeds you're getting during day and night time
- iPhone which model
- how many signal bar, 3G or 4G?
- indoor or outdoor

Doesn't matter prepaid or postpaid, speed are the same as long that particular plan does not have any speed cap.
You got to file a complaint to Digi so that they'll take action to solve the network issue there, or else they won't know, and the network problem would stay as it is.

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If you wanna know which telco has stable (and fast) Internet other than the telco you use now, you try ask around your friends and family that use other telcos like Maxis, Celcom, U Mobile, Unifi Mobile, and find ways to bring them (bring their phone) to the place that you want to have stable Internet, so that you can go test out the telco's network, such as performing speed tests, web browsing, stream videos, etc. And compare which telcos provides the Internet connectivity as what you expected in the area.

Or else, buy a prepaid SIM cards from different telcos and compare the network performance by yourself. That is better than having 1 year contract which you don't know now whether its network is as good as you expect or not.

Since you got the budget of RM45/month and 15GB data usage seems sufficient for you, could you also share on what u usually do on internet, like would you be Facebook-ing a lot, watch YouTube a lot, or just light usage like messaging apps such as WhatsApp, or what?

You can port out to other telco while remain your current Digi number, so u don't have to worry about losing ur current phone number. You can submit the MNP (mobile number portability) application via online, and almost all telco can do that, except Yes 4G, which u can only do it in selected Yes Store that accepts MNP applications.

I understand that you don't need a super high speed internet, just fast enough ones will do. I was wondering, you use WiFi hotspot and connect to ur computer? Have you tried USB tethering, connect ur iPhone cable to ur computer's USB port, then select "Bluetooth and USB Only" (later u turn off Bluetooth if u don't use it). It'll work just like connecting a LAN cable to ur computer so that you'll get to use stable data connection, instead of relying on WiFi, which is susceptible to interference. You try this method and see if the network improves or not. Cuz getting a pendrive shape modem ones (usually called 4G LTE dongle modem), you need to spend another amount of money to buy the modem, and can only used on computer by inserting to the USB port. And it doesn't seem so "financially efficient" if you just plan to use it only for few months.

Anyhow, I strongly advise u to complain this to Digi, explain to them how this issue has greatly affected how you're doing ur work. They'll try to solve the issue as fast as they can. Remember, you are a paying customer, you have the right to file a complaint if you're not satisfied with the service. Practice your consumer rights. 😉

If you wanna buy multiple SIM cards to try the network, you can insert the sim card to ur iPhone, use USB tethering to ur computer to use and test the network. So not necessary have to buy a 4G dongle modem.

So how's the Yes 4G network performance?
And are you sure that you didn't confuse with RM1/day Konfem Unlimited plan. That plan doesn't allow hotspot tethering, and has speed cap of 4Mbps.

Perhaps, have you try to restart your phone?

They have to remain the double quota, to make their plans competitive with other prepaid telco plans. Also, it is a marketing tactic to encourage more people to subscribe to the plan as soon as possible, cuz they never know when will it ends, even if Yes 4G intent to make it an ongoing and permanent offer.

This is a common practice among many telcos, but not every offers like this are permanent.

Which model of iPhone are u using?

yeah. Indeed. It does not work on my old iPhone 6 Plus and iPad mini 3.

It works on my iPhone SE and iPhone X perfectly.

I think if one has an iPad cellular the newer models that can support yes 4G, it will make quite a perfect data plan for ipad.

which model of huawei modem are u using?

u are using the rm30 unlimited 4Mbps or 10GB + 10GB with uncapped speed?