As far as I know, Mac OS is supposed to always prefer 5 GHz over 2.4. But in practice, it seems to also like to re-connect to the last AP, instead of the strongest signal, which then may connect using 2.4GHz.
Perhaps reducing channelization or power levels may help.
This year I just turned off 2.4 GHz completely on our main SSID, and that simplified things. I've left it on the guest network, on the off chance that some device doesn't support 5 Ghz, and needs 2.4 GHz, but those devices are rather rare these days.
See:
https://support.apple.com/en-us/HT206207Selection criteria for band, network, and roam candidatesmacOS always defaults to the 5 GHz band over the 2.4 GHz band. This happens as long as the RSSI for a 5 GHz network is -68 dBm or better.
If multiple 5 GHz SSIDs meet this level, macOS chooses a network based on these criteria:
- 802.11ac is preferred over 802.11n or 802.11a.
- 802.11n is preferred over 802.11a.
- 80 MHz channel width is preferred over 40 MHz or 20 MHz.
- 40 MHz channel width is preferred over 20 MHz.
