Port forwarding issue with 4G Modem in Bridge Mode and WiFi router

I’m trying to set up public port forwarding so that external requests get forwarded to my Home Assistant server.

My setup is:

Internet
|
4G Modem with SIM Card (Huawei B535-333)
|
Linksys Velop WiFi Mesh Router
|
Raspberry PI running Home Assistant

The 4G Modem is set in Bridge Mode, and all router functions seem disabled.

Port forwarding is set on Linksys router:

Accessing the Home Assistant server works fine if I use a local address on the 8123 port.

I tried to use https://www.yougetsignal.com/tools/open-ports/ to test what open ports I have, yet nothing seems to be open on my IP (tried 443, 8123, 80, etc)

I’ve also checked for logs on my Linksys router but there are no “Incoming connections” logged or “Security” events. I’m suspecting that the 4G Modem is dropping all incoming connections, but I can’t see that anywhere. Is “Bridge Mode” a valid way to setup the connections between these 2 network devices?

This is what the 4G modem admin interface says regarding bridge mode:

Bridge Mode

In bridge mode, only one device can connect to the
Internet at a time. Note:

1. In bridge mode, you can only access the Internet using mobile data.
2. If you enable bridge mode when not connected to the Internet, or manually disconnect from the Internet in bridge mode, you will be
   logged out of the page. You will need to manually assign a static IP
   to your computer to log in again.

I’m not sure what I’m doing wrong or how I can further debug the issue.

Here’s a diagram of the setup if it helps: