Repeater stops working the minute I turn AP on it

I've setup Archer C2 V1 as a repeater (using the official guide for it) and it works fine as long as I only connect to it with a wire. The minute I create a wifi access point on the device, Internet access is lost - both through the wire and wifi - I can't get a DHCP address. It happens on 19.07.3, current snapshot, Gargoyle 1.12 and Archer C20 V1 which is near identical to C2.
Is it a known problem with mt76 or am I missing something?

Is the AP part configured on the same channel as the STA part?

Of course.
I forgot to add that I only use 2,4 GHz radio.

Travelmate doesn't work either.

Travelmate Status (Quality)
connected (net nok/100)

Also I just realised that the driver for MT7620A is rt2x00, not mt76. Is that correct?