An AS is a collection of networks (IP v4 or v6) that is managed by a single entity, usually an ISP. They have a common routing policy to connect those networks to and from the rest of the internet, peering with one or more other ASs.
It is unusual to block an entire AS, which may represent dozens of IP networks (rather than just blocking a specific IP network or address), but if it is associated with TOR networks (or other potentially malicious traffic), or has a significant number of blacklisted network addresses (e.g. NK, China, Russia), it's possible that the entire AS is blocked. Bear in mind that PPRuNe is protected by Cloudflare, which gathers such threat information and acts accordingly.
Can you tell us what the VPN IP address you get on your mobile is? You can use a what's my IP website to get that, if you can't see in in the device settings. It would be interesting to see what country & network that egress point is in. Perhaps you need to choose your exit point more carefully!