Nextcloud box disconnects itself

I have purchased a nextcloud box to use with a raspberry pi 2.
I have set it up without a problem, and have accessed to it with an IP adress
but after a while; the connexion stopped working.
I have used the nextcloud box wiki to start from scratch; redone; was able to sucessfully log into the box, and then after a few hours, it disconnected again

Can you connect a screen and keyboard? Or can you still login via ssh? Would be nice to know, if the network interface was disconnected, changed IP, or if the webserver somehow crashed…