Zyxel 7101 (outdoor) together with Zyzel 5101 (indoor). No internet connection.

I've got two pieces of hardware from Zyxel: the 7101 outdoor router and the 5101 indoor router. When I put my SIM card in the 5101, I get a working internet connection. However, when I put the same SIM card in the 7101, I get nothing. That is, from the admin home page on the 7101 I see:

• Cellular Info
  • Mode IP Passthrough Mode
  • Status Up
  • IP Address 10.194.225.227
  • Primary DNS server 62.140.140.251,62.140.138.233
  • Access Technology NR-NSA EN-DC
  • Signal Strength-109
• System Info
  • Cellular WAN: 5G is green (!)
• The LED lights are flashing though (green/amber).

It's a Dutch SIM card, want to use it in Germany. Obviously, I have put roaming to on. And the pincode has been entered succesfully.

Couple of questions:

• Is there a proper manual that describes which settings need to be made on the 5101 (latest firmware) to use it as the internal router, which connects with the 7101 (as per the design so to speak, as the 7101 is not for direct internet access). I've tried IP Pass Through, but the manual is not very clear as to how this setup is supposed to work.
• What is wrong with the 7101 (latest firmware)? Given the admin home page above, it looks like there is connectivity.

Any help in the right direction would be much appreciated.