What would cause my TV and my wireless modem to burn out?

Have the circuit checked for a loose connection in a shared neutral , which is one white wire from the panel that handles two circuits. The problem also can be out in the utility wiring.

You can try to measure the voltage at the TV, it should be close to 120 volts +/-10%. You may not find it high as it can come and go.

Also possible the grounding system is faulty.