For Traefik to create an individual LetsEncrypt TLS cert, you need to create a router with the domain name.
Traefik loads dynamic config via providers, so you need to add the new router (or updated rule) to a dynamic config file, to container labels, a http service, redis or whatever providers are available (doc). Traefik will then pull config updates.
From a few posts on the forum (example), there seems to be a REST provider to actively push config updates to Traefik, but it is undocumented, a leftover from v1, so not really recommended.