CM1000 - Optimum/Altice blames speed issues on modem, won't update firmware

Paying for 400 Mbps from Optimum/Altice. (It is literally our only option for ISPs at our building - I have spent three years trying to get Fios on our block or roof access for a MESH dish. Only option.) Despite paying for 400Mbps, we get 20 on a good day. This past week, the connection has frequently slowed to effectively nothing, and when we have a connection at all, it's testing at 0.3 Mbps. Slightly faster if I bypass the router, but then it's speeds of 15 during a good hour/3 during a bad. 

Optimum sent out a guy a few days ago who said our wires are fine, it must be the modem.

After that, I called a few times to ask if Optimum could update the modem firmware, as it seems to be four versions out of date. 

I've spoken to several call center people and one "level two" support person. The "level two" guy spoke very little english and did not know that router and modem were different things. Or what is firmware. It's not even that they can't/won't update the firmware. They have no clue what I am saying. I don't know how to get anyone on the phone who is even able to understand what I'm asking. They just read a script that ends with $80 for a tech to come out. Every person I've spoken with has told me something different, too, from "I can see that you're not getting anything close to the speed you are paying for," to "from what I see you are getting good speed," to "we support that modem and it's not a problem" to "we don't support that modem and unless you rent from us, no internet for you."

Do I have any options with these clowns? At all? I'm back to work on Monday.