If you are going to have only one device running or want A COMPLETELY HACKER FREE setup! Use this SUBNET MASK: 255.255.255.252 that will allow ONLY the MODEM and THE DEVICE you are using to be allowed access to the modem!
If you want to switch between devices! Turn off the modem have cables in place for devices!
TURN THE MODEM OFF THEN ON! / REBOOT Have the device / PC / laptop /phone on! And the device that is on and plugged in will be the ONLY device able to access your modem!
I live in a rooming house full of HACKERS so I can't use WIFI! If you are in a situation where this is occurring or could occur! NEVER have the WIFI on your MODEM ON!
Don't use ANY WI-FI mice, keywords. Etc
A hacker WILL access your modem via the mouse wifi CHIP! And alter the modem so that you don't even know of see they are using your modems WIFI!
They can alter the device so that even if you turn the wifi OFF or ON it is always ON!
It took me a year of tweaking and a simple search on GOOGLE to find the correct netmask 255.255.255.252 for 2 hosts only!