try swap sim slot

do u received sms on the prepaid sim to change sim card, they have upgraded their network

and seeing from the signal from simcard1 and simcard2 not the same it seem simcard2 haven't be upgraded to support the upgraded network yet

my suggestion would be change the 2nd simcard

If your prepaid is on a finite fixed plan, i.e. 8GB, it will be in full speed. If you’re on any infinite or unlimited plan, it will have some sort of QoS imposed. So far this is what I know.
You can try to subscribe an hourly unlimited plan in prepaid and try it out again.

Just in case, is the APN set correctly for both SIM slot?

Try contact the customer support. Maybe they wrongly throttled your line due to ACCEPTABLE USE POLICY.

Try below method.

Off Sim 2  (prepaid) -> restart phone -> speedtest
Off Sim 1 (postpaid)  -> restart phone -> speedtest

or

Put 1st sim (postpaid) to sim 1 card slot -> No sim card at sim 2 card slot -> speedtest
Put 2nd sim (prepaid) to sim 1 card slot -> No sim card at sim 2 card slot ->speedtest

I noticed there's differences between both sim signal strength. Could it be connecting to different tower or different frequench band?

If no pass or add on, how you’re going to use the data service?

use 5g lor baru laju

You look at the reception signal for SIM1 and SIM2 also different already

I also noticed this for mine but mine are both prepaid

One can be 4 bars signals while another only shows 2 bars

Sometimes need to toggle airplane mode only both will show same strength

I suspect could be connected to different tower or telco (since now 4G can use unifi own network + maxis + celcom*)

*CELCOM not sure still have contract with unifi or not

2G I can confirm because my house will jump between CELCOM and MAXIS

Well I guess.. pay cheap get cheap network

Today again, CELCOM 2G full bars but SMS not coming in... switch to 4G then received

Your issue only the telco can answer you as I am anti postpaid

Just hope your plan has no contract.. then you are tied for 1 month then ciao lo

Just assume Mon-Fri excl PH

No idea bro as I said I no see postpaid, my unifi mobile prepaid lasted me from launch and topped up RM10 that's all

Your postpaid plan unlimited quota or with high amount quota plan?

It's to avoid abuse and turning your line into a dedicated fixed line.

Prepaid very fast because you buy expensive quota which cannot expire. Faster consume means you buy more and spend more.