nice you said you have Ext version. This is important.
Can you try changing channel?
If Tx is good I don’t see there is a reason repeater is that bad. I think you need to try to test the real speed using your phone when testing as ap. If you only test the signal strength that may not give a correct information.
For external antenna version we have experience that people attach and detach antenna when the devices is powered on and this damaged the radio rx.