Server only let 1 player online

So recently, me and my friend started to get problem. Everytime my friend log in the server without me online, he plays fine but when I login (the server has 2 player online) he get kicked out for "Timed out"????

We have tried everyway possible. We already khow the problem is not from Internet or something like that.

My server: PaperMC 1.16.5 no plugins

Please help...

Quote from AlexSV

What launcher does he use?

Can you please share the server log?

Tell him to try the Dyn IP, more info here: https://aternos.org/connect

He use SKLauncher not Tlauncher

server log here: https://mclo.gs/nttyJM1
We tried both dynamic and static ip, not work either

Quote from AlexSV

Try changing your dns

no... nothing happens