I see that your page is using a valid Let's Encrypt certificate now, isn't it? Do you still have any other issues?
About this error:
2020/01/20 11:52:33 Could not obtain certificates:
acme: Error -> One or more domains had a problem:
[www.fitroyownsit.com] acme: error: 400 :: urn:ietf:params:acme:error:dns :: DNS problem: NXDOMAIN looking up A for www.fitroyownsit.com, url:
Probably your domain name was not pointing yet to the external IP of your new instance. Domain name changes usually take some hours until they are propagated. In order to know the propagation status, you can use a page like DNS Checker
This other error:
2020/01/20 11:55:45 Could not obtain certificates:
acme: error: 429 :: POST :: https://acme-v02.api.letsencrypt.org/acme/new-order :: urn:ietf:params:acme:error:rateLimited :: Error creating new order :: too many failed authorizations recently: see https://letsencrypt.org/docs/rate-limits/, url:
It is just that you hit a Let's Encrypt limit to avoid abuse of their services. After waiting some time you should be able to use lego client again.