@JackAlaska Set up the new router as a wireless access point: https://www.tp-link.com/us/support/faq/1384/ You might have to use both the Tether App and the web interface.

@Riley_S For the OP would the TL-WPA7517 KIT support mesh over power line with the AX21? On its page OpenMesh is not mentioned but does show in the comparison.

@David-P Putting the 2nd router in AP mode and using the same SSIDs and settings is fine for most. I've been doing it for years without issue. Getting a mesh system by itself won't make your wireless...

Just to close this out the support team gave me a better solution: Disable "Amazon Wi-Fi Simple Setup" under the advanced wireless settings.

@Kevin_Z Apparently the Arris uses 3 IP addresses. One isn't really its address but the public IP of the upstream ISP that it passes through. The second IP is 192.168.100.1 that is used for the web...

@NotAnotherZeke: Working with TP-Link support they found that the Arris uses an IP of 192.168.0.1 in addition to 192.168.100.1. Apparently my AX21 also uses 192.168.0.1 internally for the wireless...