@Fredtx On your clients Ubee modem/router try:
Username: technician
Password: C0nf1gur3Ubee#
All this login does is allow you to configure more options on the CPE end via GUI- one of which is the "bridge mode" option. I add the former merely to save you a frustrating phone call.
Having been a TWC customer (residential AND Business,) - and now once again a residential customer (Spectrum/TWC) living in South Carolina- I have felt (and to some extent-STILL feel) your pain and frustration. During the course of reading this thread I ran a tracert on my end (EdgeRouter-4 > "Bridged" Ubee M/R) just to check and got inside private 1st hop and outside public 2nd hop.
When I had the TWC Business Static- I want to say I remember the entire support mechanism being an entirely separate entity. That being said- me sharing my experience isn't solving your issue so I'll check out now. Best of luck, bud.