Hello,
inside your log:
DNS problem: NXDOMAIN looking up A for nas.mydomain.com - check that a DNS record exists for this domain; DNS problem: NXDOMAIN looking up AAAA for nas.mydomain.com
The A or AAAA record for your domain nas.mydomain.com is missing.
You just have to add an A record for the domain nas.mydomain.com that point to the IP of your Traefik.
It's not a Traefik configuration problem, it's just a DNS configuration problem.
@bluepuma77 It's better to use the TLS challenge over the HTTP challenge but in this case, the DNS challenge will work, it's just a problem with the DNS configuration.
Using multiple resolvers is a good practice.
The pretty small difference between 1.1.1.1 (Cloudflare) and 8.8.8.8 (Google), in the context of a DNS challenge, has no impact because the resolvers are not used for the validation of the challenge but just for waiting for the propagation before asking to Let's Encrypt to check the TXT records for the challenge.
And Let's Encrypt uses its own targeted DNS servers (not related to resolvers and Let's Encrypt doesn't provide any information about those DNS servers).
So technically is better to use the slowest instead of the fastest.