How to best configure dual radios: AVM Fritz!Repeater 1200

radio0 is Atheros IPQ4019 802.11bgn 2.462 GHz
radio1 is same, but 802.11nac 5.200 GHz

I'm not building a Mesh yet, so both radios can be open for users.

For optimal performance and coverage (2 rooms, separated by a wall), should I:

a) enable both radios, same SSID, let user devices figure it out
b) like a), but different SSIDs, any point in letting users choose?
c) disable one radio, in case they interfere with each other? (Pardon the newbness of this idea, maybe.)

Current experience:

  • room wall kills 5 GHz to the other room, and devices are not able to autoswitch to 2.4 GHz - instead the connection simply dies with no clear information (to the user) why

  • I separated 2.4 and 5 GHz radios into different SSID-s, now 2.4 GHz seems to work well enough across rooms and is the default connection target

  • those in router room can manually choose 5 GHz SSID

Ideally, it'd just be one SSID, and devices would be able to autoswitch, but not sure how to accomplish this or what exactly goes wrong. I thought this was a solved problem.

Also, Will extending the router antennas to rooms be a better roaming solution? seems to have a bunch of relevant information.

Roaming is ultimately a client decision, by now most clients usually do the right thing, but there may always be stupid clients. There's nothing the router can do here. Yes, client steering (using DAWN or usteer) can nudge them by disassociating clients deemed to be connected to the wrong BSSID, but that won't really help if you're already physically out of range of the 5 GHz BSSID and the client not noticing the change (as it would be the 5 GHz BSSID's job to disassociate, but they can't do that anymore, if neither devices are seeing each other behind the wall).

The only thing you could do, would be:

  • replacing the clients with smarter ones (driver updates might already help here).
  • killing the problem with hardware, more APs, killing the deadspots so you'll retain good reception on both bands at any place in your home.