Some of my friend's can join the server but after join it keeps disconnecting and lost connection

I created server on Aternos with 1,17.1 Version, some of my friend can join without any problems, but one of my friend can join but it keeps disconnecting and then lost connection. I tell him to share his screen so I can see where is the problem. He can join but there is no blocks, and he keeps falling and then he disconnected. I tried a lot of solution like checking his versions not to get mismatching version, check his firewall, check his connection, restart the server, but nothing really works. I need some of your assistance.

Thanks in advance