Router keeps going unsecured and losing connection.

I have had service for about 6 weeks and this is about the 3rd time this has happened . Our network is secured at one point and then it goes unsecured for days leaving anyone to join, also when you are on the unsecured network it kicks us off and the modem basically restarts itself about every 15 minutes, I have contacted xfinity multiple times about this and all they do is restart the modem on their end and say that there is no problem, after about 3 days the modem is up and working, this is really getting frustrating I don’t want to continue to pay for a service that halfway works .