Do a Whois on the IP address (ex. https://whois.arin.net/rest/net/NET-99-86-0-0-1/pft?s=99.86....) and you will find who it's registered to. All of the different IPs belong to AMAZO-CF. This is almost certainly Amazon CloudFront, providing different endpoints (with different IPs) around the world depending on where the resolver is.
You're getting different IPs depending on whether you use one network or another, or one DNS resolver or another, because that's how CloudFront works (in this configuration).
WRT the certificate errors: It's of course possible that one endpoint out of many could have had a problem serving a certificate, but CloudFront is a pretty reliable service; it's likely if there was a problem with one endpoint it would have been happening with all of them. The most likely reason for certificate errors for one person in this case is the problem was on the user's end.