This was happening before the upgrade and because of it I actually did a wipe and fresh install of Monterey.
My guess would be a failure of the wifi card. At least in 5ghz.. it might work ok on 2.4ghz on the phone.
Anyway we can check by doing a few things.
1/ What model is your airport router? How old is it?
Just as a starting point.. any airport older than 5 years it is time to update regardless.
2/ Save the current configuration.
Using the Airport utility in a Mac.. open the airport by clicking on it's icon. Then click the edit in summary.
On the top menu select Export Configuration File.
Now reset the Airport to factory and reconfigure it.. this time do not use ANY filters.
Do use only short names with no spaces and pure alphanumerics.. as mine shows.. Time Capsule is tc5e.
Same for wireless.. short no spaces and pure alphanumeric.. same wifi name as the Airport if you like.
Password for WPA2 personal .. pure alphanumeric with say 8 characters or more. Important mixed case and numbers only.
Save and load all that..
Does the Mini now work.. if so you won.. the issue is Apple changed to SMB protocol and airport is still stuck with SMB1 which is deprecated on ALL OS.. Apple gave up routers in 2018 and are not updating further.
Does the Mini still fail.. if so you can reset the wifi with no password at all.. just to briefly check if that works..
If it still fails my guess is the wifi card has issues that are not going to be fixed.. you will either need to replace the wifi card in the mini or use ethernet.
Or get another brand router and see if it works. i.e. one that has modern up to date wifi.. preferably wifi6 with WPA3.
You can go back to the Exported Configuration if you wish at this point.
NOTE MAC address filtering adds NO extra security over WPA2 personal. But causes lots of issues on your side.
You can try running the Apple hardware check on the mini but previous experience suggests it is not over helpful.
You can set a fixed IP on the wifi in the correct range for the airport.. with correct DNS and Gateway (router in Apple speak). See if that works.. DHCP can fail.. where a manually set IP does work.. occasionally.
If you cannot plug the mini into ethernet let me suggest you do it with another Airport..
Airport setup in Extend wifi mode.. this will work just as well as the internal card in the mini if not better. Our term for it is wireless bridge.. so you are using a second airport to bridge from wifi to ethernet.
