You don't have to explicitly mention which certificate you are going to use.
If the valid configuration with certResover exists Traefik will try to issue certificates from LetsEncrypt.
If the TLS certificate for domain 'mydomain.com' exists in the store Traefik will pick it up and present for your domain. If there is no certificate for the domain, Traefik will present the default certificate that is built-in. This is the general flow of how it works.
I would recommend reviewing LetsEncrypt configuration following the examples provided on our website.
You can also share your static and dynamic configuration.
Thank you,