I think it's actually unifi unfairly clamping down the speed of maxis as they use their infrastructure

I've seen this myself where I'm on Maxis Infrastructure I get 38Mbps on a 30Mbps plan but my friends who are using Maxis on Unifi Infrastructure get only 28Mbps

Are you using wifi or ethernet?

Open up device manager (just type Device Manager into windows search)

Find network adapters and open up the drop down menu

Then, look for stuff like "Realtek" "Qualcomm Atheros" "Mediatek RaLink" "Broadcom" or just "Generic"

If you see "Realtek PCIe FE Family Controller" then your ethernet can only go up to 100Mbps, and that's where the bottleneck is

As for the wifi, try searching online to see what max speeds it supports, most manufacturers cheap out and put only wifi 4/n
WiFi 4/N depending on your configuration can go from 72Mbps to max of 300Mbps, which might be why your speeds are slow

Alternatively, take a screenshot of that menu and we'll see if there's any culprits that may be causing your slowdown.

That should have no issue getting 300Mbps

But you did mention plugging in via cable directly that only get 100Mbps, make sure your ethernet adapter is capable of Gigabit and you're using an ethernet cable of Cat 5e or higher

if both Wifi and Ethernet you only get 100Mbps, then please call Maxis or MCMC to complain, maybe they've set the wrong speed cap