I have tried various SD cards, PSU’s, new images, and Pi’s and none seem to work properly. Even if I connect to my network via a cable, I can’t see the player in my file manager otherwise the other option would be to use a router in my car with the player, however I can access M2P using the ip address web page.
If you connect to your network via cable, the access point cannot be established in standalone as it is connected to a network.
What file manager do you mean? You should be connected to the AP with your phone or tablet, then you can use the web interface via browser or any other app for playback with Squeezelite, Shairport, etc.
This might be an issue with your smartphone. See if there’s an option to connect to networks with internet connection or „stay connected“ or something like that.