RBR50/RBS50 dropping internet on 2.4 GHz only (affects iAqualink device)

I've been using a mesh system with RBR50 router and two RBS50 satellites for about a year. A month ago devices suddenly started losing internet connection. This has affected only Sonos and iAqualink ("smart" device that runs a pool/spa system). Those are also pretty much the only things that ever connect to the 2.4GHz band. I have gotten Sonos to work but iAqualink cannot connect to the internet at all, except for about 10-15 seconds if I reset that device. Their own tech support ran through all troubleshooting and replaced their device, but the new device is having the same problem and they say it must be Orbi.

Orbi is in AP mode, ISP is AT&T Uverse. AT&T replaced the modem, I turned off its Wi-Fi. I factory reset the Orbi. It was running in router mode for a while without me noticing, so the problem has persisted in both router and AP mode. I set 2.4 channel to 11 and set SonosNet to 1. That and a complete reset of the Sonos components seems to have fixed the Sonos problem, with a couple brief drops in internet access. Again, nothing on the 5GHz band is affected even for a moment.

The Orbi app says everything has internet access, including the device that does not actually have internet access.

Firmware is 2.3.5.30. Based on my searches, the firmware is unlikely to be the problem.

Every device in the house (about 20) is connected to the network. Every device except 1 now is able to access the internet.

Sorry if this has been covered elsewhere. I searched these boards for about a week and didn't see anything that quite fit this situation.