You probably want to change DNS provider, as this is a rather silly limitation from their part. It's perfectly fine and allowed by RFCs to have multiple TXT RR for the same name in a DNS zone.
As Rudy already said, you might also want to change to the http-01 challenge, but you wouldn't get a wildcard certificate then (as that's not allowed with the http-01 challenge).. Do you really require the wildcard certificate to begin with?
If you reaaaally need to use the dns-01 challenge, moving DNS provider also helps with handling the challenge itself, as there are multiple DNS providers out there with an API Certbot can interact with. See e.g. DNS providers who easily integrate with Let's Encrypt DNS validation for a non-comprehensively list.
Edit: Hmm, it seems Dyn is already on that list. It seems Lego is supported, so you might be able to use the certbot-dns-multi Certbot plugin, which uses lego under the hood, as a DNS plugin.