With a 15 minute disconnect cycle the problem is not likely within your control.
Line diagram:
Spectrum (ISP) -- Coax cable ---> Arris TM1602 modem ------> Router (?) ----> wired devices and ~~~~> Wireless devices.
Correct and edit my line diagram as necessary.
Does the following link provide the applicable User Guide for your modem?
https://www.top4runners.com/computer/Arris TM1602 User Guide.pdf
Do verify that I identified the correct User Guide.
======== FYI ==========
MDD:
https://internet-access-guide.com/mdd-message-timeout/
TLV-11
https://internet-access-guide.com/spectrum-tlv-11-illegal-set-operation-failed/
Start by calling Spectrum to see if they can find anything from their end.
In the meantime check everything that you can on your end - no loose wires, power plugs, etc..