The cluster is public and correct. An error often made with clusters is to think that they have an IP address. They do not. They have 2 types of DNS records TXT and SRV. The TXT provides connection strings options and the SRV point to the initial hosts to contact for the cluster. These hosts will resolve to IP addresses.
See the DNS records for the said cluster:
;QUESTION
cluster0.rcawijs.mongodb.net. IN ANY
;ANSWER
cluster0.rcawijs.mongodb.net. 60 IN TXT "authSource=admin&replicaSet=atlas-uftgjk-shard-0"
cluster0.rcawijs.mongodb.net. 60 IN SRV 0 0 27017 ac-uk0pdt4-shard-00-00.rcawijs.mongodb.net.
cluster0.rcawijs.mongodb.net. 60 IN SRV 0 0 27017 ac-uk0pdt4-shard-00-01.rcawijs.mongodb.net.
cluster0.rcawijs.mongodb.net. 60 IN SRV 0 0 27017 ac-uk0pdt4-shard-00-02.rcawijs.mongodb.net.
Try again with Google’s 8.8.8.8 or 8.8.4.4.
You might also have the wrong python module for SRV.
If it stills fail with the SRV, try to use the old style using the host provided in the DNS response above.