Use an HTML5-compatible browser (test compatibility at html5test.com).
If access fails despite compatibility, open a case with Customer Support Team.
Ensure your network team whitelists these URLs:
https://appstream2.us-east-1.aws.amazon.com/*
https://themes.us-east-1.amazonappstream.com/*
https://applications.us-east-1.amazonappstream.com/*
If a firewall port rule is required then supply 443.
More generic patterns can be used to allow for more BMC flexibility when managing AppStream hosts:
https://appstream2.*.aws.amazon.com/*
https://..amazonappstream.com/*
Cannot do IP based rules - this is a Cloudfront/Elastic IP address that likely rotates for security purposes and is managed by AWS and not BMC.
If a stricter policy needs to be put in place based on URL filtering/sniffing because of the gateway error they received:
https://themes.us-east-1.amazonappstream.com/508639275959/*
https://applications.us-east-1.amazonappstream.com/508639275959/*
Do they have WSS (Secure Web Sockets) firewall protocol rules in place?
If so, then the following would need to be allowed:
[wss://*.amazonappstream.com/*|wss://%2A.amazonappstream.com/*]