I did not find how to setup intra-device SSH on the suggested page. Key-based logon works on both devices via Putty. Another thread Controling WiFi from another router did suggest using the below command to create the necessary key linkage between devices.
ssh root:password2@router2
Running the above command resulted in 3 requests for the password before disconnecting. This seems odd given that the password was provided in the command (not shown above but 'password2' was replaced as was the IP for router2).
ssh: Connection to root:password2@router2 exited: Remote closed the connection
Uncertain whether this was the expected result, I checked /root/.ssh/known_hosts and discovered that router2's key ID had been added although in ed25519 format rather RSA. Apparently dropbear's standard key format is ed25519.
Still trying to figure out the rest of this process, i.e., Router1 has no file id_rsa in /root/.ssh/.
Also, and this was a surprise, the DHCP and DNS firewall rules for the Guest Wifi disappear permanently whenever the Guest Wifi radio is disabled. Thus, playing with the radio's Enabled/Disabled button has undesired consequences. May be better to Stop the Guest interface except that that leaves the Guest Wifi radio transmitting while concurrently removing the ability to connect. So, that seems like taking a wrong turn off the highway. Perhaps better to re-instantiate the firewall rules each time.