Surprised that no one suggested looking at the Ethernet LEDs before. Seems like step 1 with this sort of problem.
I am not familiar with MTU settings, its never been a problem that I had to deal with.
The WAN MAC might matter in terms of getting onto the Internet but I would not expect it to matter in terms of Ethernet level connectivity to a cable modem.
At one point you said that the Surf SOHO WAN port happily connected to a LAN port on your old modem. From there, were devices connected to the Surf SOHO able to get online? If so, treat the Ethernet cable plugged into the WAN port of the Surf SOHO as gold and use it to connect to the cable modem. Maybe you are dealing with an iffy Ethernet cable. You seem to have tried pretty much everything else.
Another point: is the Surf SOHO status light in the front solid green?
Dont give up on firmware 6.2.2 so quickly. There are some quirks with Ethernet devices on 6.3. Say on 6.2.2 - it increases your odds of success. Lesson learned the hard way.
The idea of connecting a switch between your Surf SOHO and cable modem was a great. I know it didn’t help, but I admire thinking out of the box.
The computers and VOIP devices are a mix of 100 and 10MBps (no gigabit). Some of the VOIP devices are connected directly to the SOHO, others are connected thru an Ethernet switch. Only the VOIP devices had a problem (during the brief initial period when the SOHO was talking to the cable modem.
If the VOIP devices have an interface, they may have error messages. That said, when I had problems with a Roku box and a stereo receiver, their self-diagnostics and error messages were useless. It is somewhat disappointing that the Surf SOHO does not report anything about Ethernet activity - at least I haven’t seen it. On the whole, it does a great job of reporting activity.
Tried that too. With the old router, I can access the cable modem on 192.168.100.1, but not with the SOHO.
The cable modem certainly has detailed Ethernet stats and very likely has an error log too. If you have to talk to the cable modem thru your old router or directly connect a computer, there is likely to be a ton of useful information there.
I’ve tried two different brands of cable modems (Motorola and Cisco). Have rebooted everything multiple times. Sometimes just the router, sometimes just the modem, modem first, router second, router first, modem second, etc. It just refuses to connect.
A nasty problem indeed. Good luck.