Me too experiencing this issue after switching Maxis Home Fibre to Unifi. I have to subscribe VPN service for my Apple devices applications download/update. Already log the report with TM, they will send technician next week.

Got the technician came over for trouble shooting. Repaired the cable, change new router, result still negative. Have told them only Apple Apps download/update having this issue. Also prove them using VPN download no problem at all. They have no idea and will update to their network team to follow up.

I found something might solve this. I this it's related to the DNS and the wifi connection. I change the DNS on my phone to the one I bought to watch Netflix, I noticed the download is faster. And so happened that I tried the download near to my router, the phone next to the router, then BOOM. All download/update finish within 10 minutes.
I try to change the DNS to auto as per router IP. The download stuck again, switch back to the purchased DNS IP, it working again. But the download slowing down when I back to my room, the router is in opposite room. I tried to test the download again near to router then download resume rocket speed.
This is really weirdo. But at least I found the way to resume high speed download although it's not convenient.

UNIFI technical team called me yesterday and told me they solved this issue. I tried the download it did perform better but not fully speed yet. Will check again with them today to see actual cause they found.

TM called me and confirmed there is some software update required their side with Apple. Now they already updated and the download for App Store resume normal. For my case, I have tried on my iPhone, iPad, and MacBook. All seem normal now. Hopefully this time they really resolve it.