Im getting the same thing with my Motorola MB7621. Had a tech out on 07/02/20, they replaced a splitter inside and the grounding block outside and said everything was fixed. I knew this wasn't correct and have continued to experience the same problems. I had this same problem a couple years ago and it was caused by a bad coaxial cable ouside of my home, either the mainline or possible a neighbors cable.
I reached out to Motoroal and received this response.
"The cable modem has sent 16 ranging requests without receiving a ranging response reply from the provider. The cable modem is, therefore, resetting itself and restarting the registration process to the service provider.
This is normally caused by noise on the upstream side and can make it so the modem's power level is not sufficient enough to transmit messages. The inability to transmit messages within the timeout period will trigger the reset and restart the registration process. A T4 timeout is a more extreme version of a T3 error.
Generally, this indicates an occasional, temporary loss of service, but if the problem persists and is increases in frequency, it likely points to a larger issue with the condition of the coaxial network outside of the home, the cabling outside of the home that will need to be addressed by the service provider."
I now have a service tech scheduled for 09/12/20, a two month wait may be too long to wait, might have to cancel and find another provider.