I have an off-grid setup with a few devices on a local network that is not connected to the internet. I can tell my iPhone to use the non-internet wireless LAN to talk to those devices, OR I can tell it to use cellular data to talk to the Internet, but there’s no config on the iPhone side to let them be both live at the same time.

Is there any magic config on a wireless router e.g. certain DHCP settings or just disable DHCP, that will let the iPhone route to static 10.x IPs on the WLAN while the cellular internet is still active?

Any “advanced network settings” on the iPhone to manage multiple NICs?