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?

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