@Zash 

The speeds being "up to" is the key when judging the performance. The real-world speed you finally get can be determined from a variety of factors. Sometimes it can be the computer, the port, the environment. The adapter may be faster on one computer, and not on another or the adapter may be bad.

Here are tips to improve wireless performance: link

What you can also possibly try is the chip manufacturer drivers: link

@Tony 

Test cases done:

1. Desktop computer, wired connection using LAN cable → reached 300+ Mbps, the real speed of my connection plan.
2. Desktop computer, placed it beside the modem, as close as possible to the Archer T2U → only reached 60+ Mbps max.
3. Laptop, placed it beside the modem as well, connected via the Archer T2U → only reached 60+ Mbps max.
4. Mobile phone, Nokia 8 (flagship) → reached 150+ Mbps since that's the limitation of the chip.
5. Mobile phone, Poco X3 (above midrange, below flagship) → reached 90+ Mbps since that's also the limitation of the chip.

So with these tests, my Archer T2U is malfunctioning?

Update EDIT: I installed the realtek link you gave and it still gave the same results.