You can try to disable band 7 when indoors.

*#2263#

Stack 1 (or 2 depending which sim you use for data)
Band disable
LTE
Tap on Band7 once (no more * beside band 7)

Try and see if it improved.

Repeat 1-3 and reset when you're outdoor so that you can enjoy 4G+.

You can try to disable band 7 when indoors.

*#2263#

Stack 1 (or 2 depending which sim you use for data)
Band disable
LTE
Tap on Band7 once (no more * beside band 7)

Try and see if it improved.

Repeat 1-3 and reset when you're outdoor so that you can enjoy 4G+.

It's quiet common for Maxis data connection quality to drop to H+ when u receiving any call.. According to the Maxis technician

I still think that you should call their Hotline to reset your data account connection

That's because Maxis don't support Voice over LTE (VoLTE) yet. All the normal calls would force phone to drop back to 3G network.

DiGi and Yes already activated VoLTE in some area. Maxis will take a very long time or will never even implement it. Who knows.

Why Band7? Band 3 is 1800MHz while Band 7 is 2600MHz. Technically higher spectrum would translate into higher throughput but at the same time it sacrifice wall penetration. So band 7 would have harder time penetrating offices walls than Band 3 which is why I recommended you to disable it altogether to force your phone to strictly use Band3. If you don't, Telco can force your phone to band 7 when they feel their band 3 is too congested.