You got it, but your solution won't work because of one detail: I can not use the DNS challenge because I am running a managed service provider, and my customers are the ones who own the domain. All I can do is ask them "please add a CNAME to my gateway", and I need to figure out everything else on my side.
That sounds interesting. Do you literally mean that I should use _acme_challenge or were you giving an example?
In concrete terms: to set up an instance for a customer, I've been asking them to set their domain with a CNAME to "gateway.communick.host", where I have traefik already configured with one TLS HTTP resolver and one DNS resolver. and I've been using the HTTP resolver one. Are you saying that I can just configure to use the DNS resolver and it will work, or do I need to tell my customers to add another record for the delegation to work?