presumably Orbot doesn’t have a setting to exclude an address from the VPN (otherwise you would have used it already)

Waait a second, what’s this then?
https://cdn.ilovefreesoftware.com/wp-content/uploads/2018/11/free_android_browser_with_builtin_VPN-05-Orbot.png

If this setting is right there, what’s stopping you from having a 2nd browser that you DON’T include inside the VPN mode for use solely to login into the wifi portal?

As best as I can see, torrc has no option to exclude an address. The closest option is MapAddress, but even that doesn’t allow you to skip an ip, only rewrite it. All requests to the tor daemon are predestined to go through the tor network, one way or another. There is a way to setup a private hidden service that loops back to a local ip like 192.168.1.1, but obviously the hidden service will not work until your wifi is already connected.

You need to do something to the ip before it is handed off to tor. Whatever method Orbot uses to emulate a VPN, you need to bypass that. Since presumably Orbot doesn’t have a setting to exclude an address from the VPN (otherwise you would have used it already), you need to figure out a way to gain access to the Android network stack (such as in a terminal where you can run ip commands, sorry not familiar with Android manual network management), and then mess around with ip route or ip netns or iptables commands to get around the VPN (if you really don’t want to turn it off even for a minute). Obviously the tor process itself is not using its own VPN to access the internet, it has access to the real network somehow, so copy that.