Spectrum Internet

Hi everyone so I had bought a modded modem from someone who came and set it up it worked fine for about 6 months then the light on the modem Sb6121 started blinking the receiving light. Then I unplugged the box and left it like that for about 3 days and when I was about to throw it away I decided to plug it in one more time and to my surprise it gave out internet again but only for like 6-7 hours and shut off again. Any ideas on how to get it back to work?
By the way the signal came back this morning and has been on let’s see how long.

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