Xfinity w Netgear CM500V modem and Synology RT2600ac router: no internet

Comcast/xfinity is my ISP. In August I replaced the comcast combo modem/router with a Netgear CM500V modem and a Synology RT2600ac router, plus another Synology MR2200ac router downstream to extend the range. This has worked great for the last 3ish months, then this week the internet connection started to get intermittent, then dropped completely. 

When I connect a pc directly to the modem, the internet connection is still great, with DL speeds around 300Mbps. But when I connect the router downstream, there is no internet connection. I tried both routers independently to see if one was 'broken', and reset everything to factory defaults, but still had the same results: internet connection on the modem but it doesn't extend to the router. The router page indicates status of GOOD for both phone and router. 

Is this a modem setting, or something on the router?  I thought this modem was certified by Xfinity, and it has worked great until now (and appears to still work great, at least when I connect to it directly).  The router has been fine too, no issues until this week. 

I saw a similar post recently by @ERS1961 , and it was solved by replacing the router with a rental unit from Comcast. I can try this, but the point of using the CM500V was so I wouldn't have to monthly pay the rental fee.