They detect lower than usual TTL of packets coming from your mobile device when it routes packets during tethering. They might also set TTL to 1 to packets going from internet to your mobile, so they get dropped instead of being forwarded further. You would need to plug the SIM card into your own modem+router combo with TTL modification rules in place to avoid detection. But then they might just block your modem by checking IMEI…
Someone gave a great workaround though - use an app that creates a local proxy available through legacy Wi-Fi Direct