Setup 3rd Party Router on Fiber

Trying to do this:

https://www.xfinity.com/support/articles/member-owned-router-fiber-internet

Read this:

https://forums.xfinity.com/conversations/your-home-network/xer10-enable-bridge-mode/690fb39b0d657631dc8c5c1f?page=4

Posting so I can DM support directly.