Windows co-op constant disconnecting

[Faye the fairy]

My friend and i have been trying to play online multiplayer, originally i hosted but they kept getting kicked out saying the server disconnected whilst things were fine for me.
We then swapped and they hosted but i then got kicked out for the same reason.
We cant always get back in, we have tried invite codes but they always say "invalid"

1. playing on windows,
2. version 1.6
3. tried compatibility mode
4. told steam to connect via IP address, this lasted for 1 in game day, they were kicked out next day

but nothing works, please help
(i have seen people mentioning firewall changes but i would prefer to avoid that for obvious reasons)

 

[Faye the fairy]

My friend and i have been trying to play online multiplayer, originally i hosted but they kept getting kicked out saying the server disconnected whilst things were fine for me.
We then swapped and they hosted but i then got kicked out for the same reason.
We cant always get back in, we have tried invite codes but they always say "invalid"

1. playing on windows,
2. version 1.6
3. tried compatibility mode
4. told steam to connect via IP address, this lasted for 1 in game day, they were kicked out next day

but nothing works, please help
(i have seen people mentioning firewall changes but i would prefer to avoid that for obvious reasons)

UPDATE
changing to IP connection appears to have worked, to an extent, the server still disconnects at times however it is easier to re-join