I have a building that has no internet, but I want to setup an old router (TP-link wdr4300) to connect to some ESP32 sensors set up to monitor soil moisture.
I can then connect to the router and check the moisture levels. I will eventually attach a Raspberry Pi with Home Assistant, but not initially. I just want to be able to check the soil moisture with my phone.
What sort of settings should I setup the router with?
Near default settings are just fine. You just won’t be using the wan. The dhcp server is active by default, you need to enable WiFi (set country code, ssid, encryption type, passphrase).
Keep in mind that your phone may not like being connected to a WiFi network that doesn’t have internet, so sometimes you have to tell it to stay on the specified network when it would otherwise drop off and use cellular data. But that is outside the scope of OpenWrt (except if you really need it, you can run the “fake internet” package that helps make the phone believe there is an active internet connection).
You cam make ad-hoc ap so phone does not drop mobile data but you need to harden it like to protect open network from passers-by.
Ad-hoc is not typically necessary, and very infrequently used. I wouldn’t recommend using that method.
If you dont have the internet on device you can keep your phones internet to do servicing, not required, you can do from IoT interface too.