camera on desk top computer and not on any of my cellphone or anything using WIFI

Sometime I can see my doorbell camera and sometimes i get a trouble with internet. I can see the doorbell camera on my desktop computer, which is wired into the modem and router, at any time on the big computer.  On my cellphone some times I can see it, and some times it gives me a screen that says something wrong with server. When I first got the system I used to het that and I was blaming Simplisafe because it said we are having server trouble. But now I find out now I can get the cameras any time even when I can't on cellphone. Something different between WIFI and direct wiring to internet, I don't know if maybe a setting needs to be changed, but it dont always do this. Oh well hope I described problem well enough that some one might know what the problem is. Please help, frustrated.   THANKS !!!