Help! Androids won't stay connected to router without internet

So we use routers with no internet as a means of syncing data between a phone app and a postgres database.

Setting a static IP address has been our workaround on older phones but the new phones this doesn't fix things.

Any ideas?? The exact message is "no internet access detected won't automatically reconnect"... An annoying android update that I'm aware of.

Sorry if I'm posting to the wrong sub