Question What cable name connects to the ONT port on modems

[Fbarajas]

Recently had internet installed but the cable that goes to the modems ONT port is way too short im trying to replace it with something longer. Can i replace it with a cat6 cable?

 

[USAFRet]

ONT indicates fiber?

More info needed.
What service, how is the modem currently connected, why didn't the install tech get this all sorted out before he left?