Not sure what the original adapter size is but have you looked on Amazon for DC plug adapter kits?
Like this:
Improving Comcast modem signal
Our internet service is Xfinity/Comcast or Verizon. Currently I have Comcast service and use their modem because I had trouble with Netgear modem compatibility with Comcast. The problem is that the Comcast modem doesn’t allow LPS. I don’t know about the Verizon modem. So, for people with modem tweaking experience, any suggestions that will make a notable improvement?
The modem is connected via house Ethernet cable to EtherRegen in my listening room, which is connected to Lumin X1 streamer via fiber optic listening room.