A few thoughts...
- Go to https://browserleaks.com/ip to see if you are somehow leaking your true IP address.
- The US server (
vpn-us-nyc1.netfree.linkaccording to the wiki) is not located in the US but in Argentina. I would imagine it's easier to detect a VPN when it's not in the same country as it's clients. Do you know anyone residing in Argentina who can check if these sites work for them?
- Perhaps the domain name is somewhat of a giveaway (even though a reverse lookup returns a different domain. Perhaps the fact that a reverse lookup returns a non-existent domain is a flag? I don't know why that would be but who knows...)
- I don't think that multiple client emerging from 1 IP address is a fair way to detect VPNs and I guess that's not how they are detecting. Nowadays many cellular client are often put behind a single IP due to diminishing available ipv4 addresses. Apparently some ISPs even do that nowadays.
Edit: Actually some sites report
126.96.36.199 to be in NY not Argentina. That sounds more likely.