Using an existing Time Capsule on a TPLink mesh network

I have read other guidance on here and have tried to get this set up but missing a key element somewhere. I have the TC in Bridge mode, wifi is turned off, and it is ethernet wired to the main TPLink unit (the same one that is wired to my incoming internet cable modem). I go to the MacBook requiring backup - it previously used this TC but its been a couple years. I select TC disk and I get a message that the old backups won't work so it will start a new one - which is fine. I tell it to go ahead, and it goes into 'preparing backup' for a more than an hour. I'm patient so that's not a problem. But when I come back after several hours, that TC disk just shows 'waiting for backup' and I have no idea how far it got before it gave up or what happened to interrupt the process.

Any suggestions? Thanks for all the folks that share their smarts here.