@ArcherC8 tried and nothing. I switched the device into AP mode earlier today, and suddenly got full speed, but on cable only. This rules out faulty cable (I tried 2 anyway). WiFi is still 2mbps.

My concern is that the device does not work properly on cable either in router mode. I don't want it to use in AP mode.

@szakee 

To go off of what @ArcherC8 was mentioned about the modem/router. Have you tried setting the modem into bridge mode so it turns into only a modem? Pretty much trying to get away from a double NAT network.

There's no modem. It's an optical cable coming into a GPON CPE, from there to a PoE adapter, from which a cat5e cable is coming out.  This cable can be plugged straight into my notebook, or into the router. I tried with a friends router (an older tp-link), both wired and wifi worked immediately on full speed. So mine seems like a faulty piece, gonna be returning.

Okay, so seems like the problem is on the provider's end. Talked to them again and it seems they do have to make some change on their side (put the router MAC into their system), which they did not tell me before...

Hopefully will happen tomorrow and hopefully will solve it.