I finally solved it! Though it isn't 200mpbs, as expected to a repeater, I went to Wi-Fi> Disable 2.4GHz. I don't understand why it was enabled in the first place.

I was just wondering why my Deco M5 is capped at 30mbps while my internet speed to the main router is 200mbps? When I connect to the main deco M5, the one with the Ethernet cable, the speed is the...