Access by using second WAN interface?

Probably a silly question, revealing I don't know what I'm doing :)
We have a second internet provider. The goal is a failover configuration, but in the mean time - let's put this second connection to the test. I configured it for interface X2 and seems to work ok. I switched on Ping and when pinged from the outside, I get a response.
Let's call this connection X2-Fiber. The 'old' connection is configured on interface X1 and lets call it X1-Coax.
For the X1-Coax interface, an Access Rule (image below) and NAT policy are configured for Remote Desktop and it works fine.
Now lets do the same for the X2-Fiber connection. I thought I had to configure a second Access Rule, but noticed group WAN Interface IP is already populated with the X2-Fiber connection. So no additional Access Rule necessary.
The NAT rule to direct Remote Desktop to the correct pc is configured to use any Inbound / Outbound interface.
I.e. I should be able to use Remote Desktop by simply specifying the X2-Fiber connections IP address in the Computer field of Remote Desktop. But it doesn't work. I'm probably missing the obvious, but can't figure out what it is….