Help Needed: Configuring Zyxel NR5101 for Bridge Mode or IP Passthrough

Hello Zyxel Community,

I am seeking assistance with configuring my Zyxel NR5101 to function as a bridge or enable IP Passthrough. My goal is to allow my Fortigate 40F firewall to handle all routing and firewall functions and establish an IPsec VPN connection (Dial-Up) to another Fortigate at a different location. I hope I'm on th right place here?

Current Setup

• Location A (Fortigate 40F)
  • LAN: 192.168.10.0/24
  • WAN IP: Static Public IP
  • Connected via: Fiber modem in Bridge Mode
• Location B (Fortigate 40F)
  • LAN: 192.168.20.0/24
  • WAN IP: Dynamic (via LTE/5G connection through Zyxel NR5101)
  • Zyxel NR5101 LAN IP: 192.168.20.2
  • Fortigate LAN IP: 192.168.20.1

Problem

I need to configure the Zyxel NR5101 so that it passes the public IP directly to the Fortigate at Location B, allowing the Fortigate to manage all network traffic and establish an IPsec VPN connection (Dial-Up) to Location A. Unfortunately, the options for "Bridge Mode" or "IP Passthrough" are greyed out, and I am unable to add a new WAN interface as well.

I am starting to feel quite frustrated with this issue and would deeply appreciate any hints or guidance you can provide to help me resolve this.

Any help is highly appreciated!

Regards Peter