Accessing WANBACKUP's internal web server?

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So I have a multi WAN setup:
1) WAN - Fiber connection
2) WANBACKUP - Netger M1 mobile router

Now they do of course have different IP's.

What I would like is to access the internal web server in my Netgear M1 mobile router, to check for usage and SMS messages (that my ISP seem to like to send at different times and all that). However the current setup, which is based on the documentation from OpnSense, does not allow me to get to the web server in my Netger M1.

So say my WAN has IP 192.168.1.1 - and my WANBACKUP is 172.16.1.1.

What kind of firewall rule, I am guessing here - after all I am kind of new to OpnSense, would I need?

From my LAN source IP to WANBACKUP gateway or something similar maybe? just guessing here since I clearly need advice before I start making assumptions and basic guessing...?

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And in the end I ended up with a firewall rule:

From LAN (interface) to 172.16.1.1 and thru gateway WANBACKUP (which is 172.16.1.1) - first in order under LAN firewall rules = success :-)