I also only have the main unit connected and I have maximum values everywhere in the house.

There is no need to plug the 2nd unit to ethernet. I've a 220 sqm house and i've only connected 1 unit to the modem. I get speeds of min 900mb. Worst case, you can connect and nothing stops. The idea of mesh is lost if you connect both units to the modem.

Yes, that was the case with me.