Internet speed slows shortly after modem reset

I have a brand new TP link BE 3600 router and an Arris SB8200 modem. I have been only getting an average of 20-30mbps download speed. I have gigablast. I reset the modem via the cox app, and for a short time get 900mbps + download and 100mbps+ upload. This lasts for a few moments then right back to 20-30mbps. New router, multiple resets, no change. I have had this issue for a few months. Do I need a technician to come out and check outside cable? What else can I do?