Hopefully it has resolved your problem and seeing as the Doorbell is in a fixed location it makes sense to disable seamless roaming. I did the same with mine when I was trying to improve the wireless signal but I think I might have re-enabled it when it had no impact on the rssi.
My Doorbell 2 is mounted to the pillars of my automated gates which are approx 20’ from the Deco M5 and 25’ from my ISP supplied wireless router. The only objects between them are a double glazed window and a slatted timber gate, even when I temporarily moved the deco to the outside window ledge the signal only marginally improved. My smartphone speedtest reported 890mbit down and 90mbit up for the same location at my pillars when connected to the Deco M5 and similar when connected to the ISP’s router.
The Doorbell 2 was getting an rssi score of 81 with the Deco and now 63 with the ISP’s router. The mesh wifi’s signal is excellent everywhere in my home and well out to my driveway, patio and garden whereas the ISP’s wireless router has many deadspots in the home and will not extend out into the patio or back garden. We use wifi calling to boost our weak GSM signal (rural area with poor GSM coverage) on my smartphone so extenders are not a real option.
I had 6 months of almost flawless operation from the Doorbell 2(even with multiple power outages during this period) before the introduction of the Deco M5 mesh system, 4 weeks after installing the M5 and possibly after a low battery forced the doorbell to shut down it did not start up again and was bricked. The replacement Doorbell 2 lasted only 6 days and mysteriously bricked itself with 63% charge left on the battery and a power outage caused the mesh network to go offline.
Slightly off topic, the Doorbell 2 is a great piece of kit when it works, I am disappointed with the wifi connectivity and the power options for the unit. In my situation there is no existing bell at the pillar but I would buy a mains voltage transformer from Ring if there were one available for the Doorbell 2, also there is no weather shielding or cover available to protect the unit in really bad snow or wet conditions though my units failed in good clear and dry weather.
The problems with Deco M5 seem to be happening to several users and the problem repeats with multiple replacements, yet there are no advisarys available from Ring or TP-Link 6 months after some posts have appeared. I hope it is resolved by disabling the roaming option on the M5 as you mentioned and hope you will post again if that changes. Meanwhile I will keep my old non-mesh wireless router in service and see how that works out.