Interesting - Might be worth contacting them also regarding the sudden DNS error.
In terms of the cluster itself, can you check the connection string matches or if the cluster is active? I was unable to resolve the one you provided although I can understand if this is because you changed it for security concerns:
** server can't find _mongodb._tcp.cluster0.rx6v7.mongodb.net: NXDOMAIN
For reference just to compare, I performed a nslookup on my own test cluster where 3 hostnames were able to be resolved:
nslookup -type=srv _mongodb._tcp.cluster0.<REDACTED>.mongodb.net
Server: 127.0.0.1
Address: 127.0.0.1#53
Non-authoritative answer:
_mongodb._tcp.cluster0.qemgxcq.mongodb.net service = 0 0 27017 ac-iceu1mh-shard-00-00.<REDACTED>.mongodb.net.
_mongodb._tcp.cluster0.qemgxcq.mongodb.net service = 0 0 27017 ac-iceu1mh-shard-00-01.<REDACTED>.mongodb.net.
_mongodb._tcp.cluster0.qemgxcq.mongodb.net service = 0 0 27017 ac-iceu1mh-shard-00-02.<REDACTED>.mongodb.net.