RBE971 V12.1.11.15: Intel AX211 client severe latency on main 5 GHz SSID, fine on IoT SSID

Thanks. Before the specific answers, one point on the interference theory generally: this system has been in this exact spot, on these channels, with these two satellites, for about 20 months without a problem. Nothing was added, moved, or changed in the room where the router is, or anywhere else in my house, for that matter. Interference doesn't appear overnight on its own, and if it had, it would affect the Linksys I placed in the same spot just as much. What happened overnight on Sept 22–23 was a firmware update, and since then it has steadily worsened on channels 36/40. A radio that worked for 20 months and then degraded over a week, in the same environment, on the same channels the satellites and a different router still use fine, is a radio with a fault.

To your questions:

Linksys test result: different, not the same. That's the key finding. Linksys Wi-Fi 6E router in the exact spot where the RBR sits, RBR and both RBS powered off, same laptop, channel 36: ping avg 5 ms, max 104 ms, 0% loss. The RBR in that spot on channel 36/40: avg 150–1117 ms, up to 16% loss. The location and the channel are fine for a different radio; only the RBR's 5 GHz radio has the problem.

RBR off / RBS off: The Linksys test above was with the whole Orbi system off. I've also tested the reverse: RBR on, both RBS unplugged (backhaul radios idle), channel 36: avg 174 ms, max 920 ms, 4 of 60 lost. Still bad, so it isn't the RBR interfering with itself via the backhaul, and it isn't the satellites.

6 GHz on the AX211: This laptop has never associated on 6 GHz to this system. The 6 GHz BSSIDs show as colocated APs in netsh, and setting the Intel driver's Preferred Band to 6 GHz didn't get it to move; it stays on 5 GHz. My iPhone (5+6 GHz MLO) and Mac (6 GHz) use the RBR's 6 GHz radio at 1,100+ Mbps with no trouble, so that radio is fine. I haven't pursued 6 GHz for the laptop further because it wouldn't change the diagnosis: the 5 GHz radio is the one that's faulty, and a Qualcomm Wi-Fi 5 laptop and a Sony TV, neither of which has 6 GHz, are affected the same way.

Two satellites: Understood on the guidance, and I can revisit that separately. But the fault is present with both satellites unplugged, so the satellite count isn't a factor here.

Everything else is unchanged: factory reset and rollback to 12.1.10.12 didn't help, channel 44 works, channels 36/40 are getting worse, and the satellites' 5 GHz radios are fine on those same channels. If there's a specific test that would help NETGEAR distinguish a calibration or hardware fault in the RBR from anything else, I'm glad to run it. Otherwise I'm hoping the case can get an engineering look rather than the "it works on 44" answer.