@Sizif I'm curious do me a favor, make this test, let's say at the factory they reversed outdoor antenna and internal 7510 has 3 setups vertical horizontal and external can you test the 3 and see in...

Could it be a factory mistake you have a 5Ghz unit packed on a 2.4Ghz case

Do you have the supplied one to test And the GrooveA-52HPn can se set to either band, does it have a setup to both at same time

Question, GrooveA-52HPn can handle both bands 2&5Ghz and are you using the new packaged antenna

This is a good read, and you will find Why some situations force installers to use coax....

@CyberTod reading I found a post between Tik & UBNT they solved by enabling WDS, on 210 it's on Client side, have you try enable or disable WDS and what about ISP router is WDS capable