I keep having bad connectivity issues with servers

I go onto an aternos server and it works fine for about a minutes, and then it becomes hard to play because i either disconnect or everything freezes(not my frames, mobs and blocks lag) and takes ages to go back. I dont think its my internet because its only on aternos that i have this problem with and my internet works fine. My sister who is in the same house and wifi can also connect and play fine. Is there any way i can fix this?

Ah, is there anyway i can find out exactly what it is then? Also I was able to play on aternos fine on this pc about a week ago.

Ok thanks, i'll try that.