

Haven’t tried this in a while (I’ve been off iPhone for 5years now), but iPhone used to allow this as follows:
If you join your home WiFi and have no default route set by dhcp, iPhone should tell you it has no internet and you can tell it to use the connection anyway.
The iPhone feature to “use 4g when WiFi is bad” should then solve for internet access and let you do both.
Just be aware you can’t have split brain DNS, so internal LAN stuff will likely need to be accessed by ip address only, not DNS based.
I’ve used airtable for these sorts of “personal use” use cases (eg: home built rocketry flight log) and I was even able to use their iPhone / Android app to enter data.
I haven’t paid a cent for it yet as I’m well under their data caps.