At least for Comcast/Xfinity, they published an RFC here which they think somehow makes this all OK. But according to the RFC it’s only HTTP. I can recall if I’ve seen their popups on HTTPS pages or not. I can think of a few ways they could try to make it also work with HTTPS that do not require MITM of the SSL/TLS traffic but I’ll have to wait until I use up 50% of my data cap for the next pop-up to appear and try to look at the code and figure out how it got there.
Here is an update. They sent a repair tech this morning. He examined everything and found nothing wrong. Of course, I was not able to replicate the issue to him and everything is working ‘fine’. Difficult to diagnose ‘erratic’ behavior … 
Are you sure you’re connecting the modem on the WAN port and not in a LAN one? Or that the wan port is configured to be also a LAN one?!
Are you using the same subnet of the modem for your lan?!
Otherwise I have no explanation…
Yes, no issue there. I’m using the right connections. As I said earlier, my system has been working fine since I installed it for over a year now. Somehow, issues started showing up once I setup OpenHab on a Raspberry 3 (about 10 days ago). At first I thought openHab had something to do with it … now it seems it’s purely coincidental … in all honesty I’m not sure one way or another.
Now, here is a thought. When a device (with already assigned IP) is switched off, and a new device is plugged in. Does the IP for the off device get somehow recycled to the new device? Would somehow this cause IP conflicts and possibly explain what I’ve been seeing? The other thing is it was either the wired or wireless (all together) devices that would loose connection, not always the same device. I know, rather puzzling …
The service responsabile of assigning IP address is DHCP.
Surely your router has it enabled…if you installed something on the raspberry that makes it work as DHCP too…yes will surely give you problems.
The DHCP service assigns an IP for a certain amount of time (called lease time). During this time it will never reassign that IP even if the device is offline…after the lease time has expired, if the device is online it’s usually renewed. So it’s impossible for the dhcp to give the same IP on different devices.at the same time…but if you have two dhcp on the same network they will surely conflict.
Yes, I know that IPs (in principle) are not recycled immediately … I can’t explain what happened … and why it happened with 3 different routers (ASUS/Linksys/NetGear) … still a puzzle, but ok, we can move on, and hopefully won’t see it again.