The first thing to do is to
disable the "5 GHz only" toggle switch, of course.
Even though there is no "2.4 GHz only" setting, with the above setting enabled your WiiM Amp will never pick up the 2.4 GHz Wi-Fi. If you hadn't this disabled yet, maybe that's all you need to do.
If you rename the 5 GHz AP only, this should be enough. If you decide to give them both meaningful new names you should be able to change to the new SSID by simply tapping on "Change".
Yes, I'm aware that renaming the SSID means potentially making changes to other Wi-Fi devices.
How close is "proximity" in your case? Did you try moving the WiiM Amp just an inch or so and let it sit for a while? What about other streaming services like e.g. the free integrated Radio Paradise? What readings do you get for WihFu strength, RSSI, and SNR?
I wonder if your issues are really just down to the WiiM Amp's Wi-Fi connectivity. AirPlay is special because (different from e.g. Googlecast, Spotify Connect, Tidal Connect or Qobuz Connect) it still routes the full audio data stream through your phone.