Multiple IP Addresses from Different Subnets on IPhone SE

Nope. You’re going to need to establish IP subnet routing, or simplify your IP network, or possibly both.

I’d also suggest consolidating into the same private address block, rather than operating in two separate private address blocks.

Pretty much any Ethernet network interface can operate with multiple IP addresses, but now you’re trying to set up routing on each interface. Alas, iPhone either expects a manual one-subnet setup, or the address and subnet data provided by DHCP. Which won’t let the iPhone directly chat with multiple adjacent subnets in parallel.

If this is multiple Wi-Fi routers for coverage or other reasons, maybe access points are the path out of this complexity. Or a better-grade firewall-router.

I understand well what you’re after.

I run multiple subnets in different installations, with a mix of vLAN and physical separation and all subnets within the same private block (including a DMZ, and at various times a trash-IoT-device isolated subnet), and all (selectively) routable via a firewall/router.

IP clients expect one address, and lower-end and embedded boxes don’t tend to be running multiple NICs or multiple IP addresses on a shared NIC or local routing based the configured subnet and designated router.

And I much prefer to run the VPN server in the firewall, as that avoids some of the common messes involved when trying to pass VPN connections through a firewall via port forwarding.

Yes, though I’ve had various cases where firewall port forwarding cannot differentiate multiple parallel VPN connections, and gets the connections tangled and disconnects. And I don’t prefer to VPN directly into a host-based VPN server running on the server that may be having issues, for obvious reasons. Nor do I prefer to work with Cisco gear, and that for various reasons.

Again, I’d get a firewall/router between your IP subnet segments, particularly if there are differing levels of trust across the equipment connected to different subnets, and as trying to connect to multiple subnets and to troubleshoot preferred paths gets gnarly on some systems.