2.4Ghz Wifi keeps dropping connectivity

Both 2.4 and 5Ghz have worked just fine for the last year; actually added a range extender so that the signal reached the back of the house (on 5Ghz). 

About 2 months ago, the 2.4G stopped working, to wit, it still was broadcasting the SSID just fine, and one can 'connect' to it (at least part way, my cellphone does fully but a Roku errors out trying to) and the cellphone has no connectivity to the internet when this happens.  The 5Ghz is unaffected, keeps running normally.  Rebooted the 'gateway' (power cycle) and it came up and ran just fine.

Yesterday, it happened again.  Rebooted again, and the 2.4G came right back up and ran fine for about 15+ hours.  Then died again.  This time, I tried the 'toggle the 2.4G wifi off then back on' in the settings, and it didn't seem to give the thing a 'kick in the behind' and start working, it just sat there.

Again, the SSID is being broadcast just fine (both 2.4 and 5G), and the devices trying to connect on 2.4G find and at least partially connect (the Roku seems to 'see' the wifi but not the 'internet' and so errors out; my cellphone 'looks' like it connects but there is no 'real' connection to the internet, zero). 

This apparently keeps happening over and over at this point, and the 'quick fix' that I found (tried above, toggling the 2.4G wifi on/off) didn't work, that the only thing is a 'full reset/power cycle' to get back running, but then it will die again in x hours/days/weeks.

Update:  This time I didn't try and get things up and running (no reset) right away, but let it sit there and stew awhile (particularly as the 5G continued and continues to work perfectly), and kept an eye on the 2.4G.  Lo and behold, about 5 hours after the 2.4G stopped working, it out of the blue started working again, and has so for about 5 hours or so since.  I'm beginning to wonder if the Comcast/Cisco 'Gateway' modem is going bad.