Comment 3 for bug 1919560

You can upgrade the charm to workaround the issue even once it's stuck.

Otherwise the manual workaround is to DELETE the rows from the mysql_innodb_cluster_metadata.routers table with the IP address matching the unit that can't create the new router. Likely in that charm version the router 'name' is set to '' but if there is more than 1 router for the same IP you also need to match the name. (Fairly sure the name sending was added after the --force fix)