A year ago (10/11/2019), I posted this same problem with the Technicolor DPC3194B modem and problems with port forwarding (IE: an absolute incapacity to accomplish this).. I THOUGHT the issue was resolved after the Comcast phone tech did a remote 'factory restore', but I guess I shouldn't have been so quick to accept that it was, in fact, "fixed'.
Within mere days, the ports I had forwarded were inoperative again - no port forwarding was achieved nor could I even access the previous configuration. And just as was explained in the OP initially, there was no evidence of any attached devices via Ethernet connection or wifi.
Apparently the firmware allows the two possible means to achieve this, which are via an attempt to configure an 'application hosting' rule, or simply a 'port forward' rule, but neither will actually function for longer than a few hours. It is as if somehow the firmware is somehow being 'reset' or the ports 'blocked' despite these port forwarding rules that STILL appear as properly configured (but cannot be accessed or altered).
Recently, because of the COVID-19 pandemic, we NEEDED to re-establish remote administration to this LAN via our remote access and management software (NetOp), but once again this is seemingly impossible to achieve. Frankly, I'm beginning to believe this modem is in some way defective since all efforts to port forward or configure a 'host application' rule AND to have it remain in place and operational for longer than a day simply cannot be accomplished, and I've been doing this type of thing for a VERY long time now.
BTW: this past week when I called Comcast Business Support about this (...again), I was told (literally) that "...Comcast does not use Technicolor DPC3194B modems in this market..."
Hmmm... Really..? Then how did we get this one, I just have to wonder ...?
This is REALLY a problem for us insofar as concerns about COVID exposure would make remote access of significant interest for those who need to 'tele-commute' to avoid potential exposure, and certainly for someone like me that is charged with management and access to perform service on the network attached computers on this very small LAN.
I can understand why the port forward and inability to view connected devices is frustrating and I'd be glad to look into the possibility of a new modem. Please click on my handle (Comcast_Gina) and send a private message with your name, the business name, the complete service address (including city, state, ZIP, suite number, etc), and the phone or account number, so we can review this and determine the next best step.