Nighthawk CAX30 Ethernet speed slower than Xfinity modem

I was using a rented modem/router for years and decided to purchase Nighthawk CAX30 AX2700 with DOCSIS 3.1. My Internet tier was 400Mbps.

Google speed test for Ethernet connected PC on Xfinity modem was 420Mbps download.

Plugged in the Nighthawk and had it provisioned via the app. Tested at only 110Mbps download on Ethernet.

Called support and they either reset or reprovisioned (probably just rest) to no avail. They upgraded me to 1Gbps tier and then scheduled a tech to come out. 

Internal modem settings had cable connection listed as "poor". The tech replaced 2-3 splitters and the connection changed to "good". There's one splitter inside ceiling space that he couldn't get to, but said it'd need to also be changed to fix the issue (I'm skeptical because of what follows below). 

His personal tool measured 2Gbps at my wall plate, but the modem continues to top out at 110Mbps. He advised that they could either replace the last splitter, once i open up the ceiling or run a separate line directly to my living room. I don't like either of these options and feel like this is either: 

A) bad modem (I'll need to return it)

B) modem wasn't provisioned properly (most likely) 

I kept asking the tech, why would i get only 110Mbps inside if his tool shows 2Gbps and his answer was that Comcast controls what max i can get based on my tier. So without saying it, he was telling me that something on the server side is [Edited: Language], but then he kept going in circles about me having to replace that last splitter or getting a new line, so he wasn't addressing the core of my question and i became extremely frustrated, thanked him for the work he did (i really do appreciate what those guys do, so this isn't a negative feedback on any way) and told him I'll think about my next step. 

I'm a computer engineer, and while I'm ignorant on the specifics of ISP's innerworkings, it just seems to me that if my line now supports 2Gbps throughput, my Nighthawk should be able to do 1Gbps with my upgraded tier (considering its internal cable integrity assessment is now "good"). Are these new modems just so high tech that they need an absolute perfect line and will trip up over one older splitter? How are other people upgrading their modems in older homes without issues? if the speed I'm measuring is exactly the same 110 as before the splitters were upgraded, it can't possibly be just the line integrity. 

Please advise/help, as I feel that I'm not getting the answers/support that i need at this point 

Thank you so much!