Terrible 2,4ghz range Cudy WR1300 V1 - 1x antenna open 3x closed circuit

Hello everyone,

i have recently bought the Cudy WR1300 MT7621 based router and installed openwrt 22.03 on it. 5Ghz Wifi is good, better than my reference TP Archer C7 V5 (openwrt 22.03). The 2,4ghz however, is without a doubt the worst i have seen in my entire life.

Here a some measurements i have taken with the phone.

Setting right next to both routers:
2,4ghz:
Cudy WR1300 -46 dBm
Archer C7 -32 dBm
5ghz:
Cudy WR1300 -30 dBm
Archer C7 -35 dBm

5 meters away, two walls in between:
2,4ghz:
Cudy WR1300 -83 dBm
Archer C7 -49 dBm
5ghz:
Cudy WR1300 -57 dBm
Archer C7 -59 dBm

In short, the 2,4ghz is pretty much unusable. Both wifis are set to the same settings, country code germany and full power. The 5ghz works great, however has serious trouble getting through brick walls. What could be the issue here?

I have opened the unit and found that all but one antenna are closed circuit(1ohm resistance), the other(#4) being open circuit. This one also seems to be the transmitting antenna for the 2,4ghz (grey cable #4). Could this be causing the problem? Should i take apart the anntena to inspect it further?

#1 and #2 seem to be the 5ghz antennas and #3 and #4 the 2,4ghz. Now i always thought, all antennas would be sending and receiving. However, on this unit, when i place aluminium foil around #4 antenna the signal drops even more. Conversly, when putting the foil around #3 and looking at the wireless scan on the WR-1300 inside luci, the surround wifis drop sharply by 20-30dBm. It seems as if one antenna is receiving and the other is sending. Can someone clear that up for me?

How should i proceed? thanks for the help.