Spectrum uses the E31U2V1 (Ubee), E31T2V1 (Technicolor) and E31N2V1 (Hitron) for Gigabit service. The Technicolor TC4400 was used in some early Gigabit installs where voice service wasn't being used, and prior to Specturm having eMTA modems.
As far as I know, Spectrum doesn't have any Gigabit approved customer owned modems, but I've heard of success getting modems like the Motorola MB8600 and Arris SB8200 activated in some markets.
What sort of problem are you having with Gigabit?
In Spectrums own approved modem list they state the MB8600 is valid for up to 400mbps....with no mention that I can find that they consider the SB8200 as approved . I beleive it is on the minifully qualified list for gig service.
I do know that a number of people have been using both on the gigabit tier for at least 2 years now.
the Netgear CM1000V2 is also certified for up to 400mbps. <----EDIT
Of the 3 Spectrum gigibit modems, only 1 allows you to access the signals page.
There are zero retail modems officially approved for gig. Two retail gig capable modems (CM1000v2 and MB8600) are only officially approved for up to 400 Mbps.
Having said that, some customers have been successful in getting their retail modems activated on gig. If you decide to go that route, 1) keep the receipt so you can return it when Spectrum refuses to activate yours, and 2) if successful in activating, expect to receive no troubleshooting support when something goes wrong.
Minimally approved means the modem is supported on the network but there are no speed guarantees on any tier.
Be very careful if you get the Netgear CM1000V2....it's a specific model and there are like 3 other model versions.....
CM1000v1= Broadcom BCM3390 processor
CM1000v2=Braodcom BCM3390R which drops certain unecessary functions for a modem like zigbee, pcie, usb, etc so the chip is a little cheaper to purchase but still performs the same for modem functions.
CM1100= is a cm1000 with 2x ethernet ports for link aggregation
CM1200= I believe is same as cm1100 with 4x ethernet ports for link aggregation.