My friends cannot enter the caves. just me

Just happened to me recently. It happened randomly to me a lot, this issue and the sudden server not responding after it kicks people and none can join. I suspected it was a NAT type disconnection, however last time it happened with my friend he was joining my caves just fine until suddnly it keeps kicking him out and we had to stop playing. Would love to know how to fix plz. Possible solutions we tried:

1- Restarting server

2-Playing with and without splitscreen

3-Disabling and reactivating dynamic content

4-Closing and starting both games again

5-Closing and restarting  internet and double checking NAT compatibility.

6-Going inside the cave and then having him join the game and try to enter caves.

Edited August 29, 2019 by Mahoood