Poor performance with Comcast's XB7 modem and XFI complete

Hi, I added unlimited data to my high speed internet service using my own modem for $30/month about 2 years ago. The day the unlimited data was added my reliable modem with excellent data transfer speeds all the way to a separate garage with 4 cameras attached 20' away from my house with the modem in my basement went to [Edited: Language]. My data transfer speeds on each camera was a consistent 70k/second on the 2.4ghz band using a surfboard modem / router prior to the upgrade. The day the unlimited data package was added my internet service instantly became unreliable. My cameras kept going offline which had never happened before. When they were connected the data transfer speeds were awful rendering the cameras useless. I reported the issue to Comcast's tech support to no avail. 

They claimed they were unaware of any such issue so no resolution was ever achieved. I then thought a new modem / router might possibly help with my newfound signal issue that apparently comes with every unlimited data upgrade. I purchased a Comcast recommended Motorolla cable modem / router combo for $170 and replaced the surfboard. No change. I wound up having to use 2 modems after the unlimited upgrade to keep my cameras working. I installed one on each side of my very small 1200sq ft home. The cameras then worked, but I only achieved 30k-50k rather than the previous 70k/second I received prior to the unlimited data upgrade. Eventually Comcast kicked my 2nd modem off the network putting me back to square one. They wanted $50/month to use a 2nd modem in my own home. That was far too expensive so I declined. 

The rep thought a good solution would be to upgrade to the XFI complete on a contract for an attractive monthly rate and Comcast would provide me with their latest high end modem / router which should solve my data transfer issues. It didn't. I experienced the same problem. I canceled the unlimited data and went back to my own modem to no avail. Same problem. I then went back to Comcast's modem / router combo connected to a Linksys ac1750 router with unlimited data to augment the poor wifi signal provided by the Comcast modem. This helped a bit, but my data transfer speeds were still poor compared to what I was getting previously. My computer is 5' from the Comcast modem with a wall in between. My fire stick occasionally buffers due to the poor signal. It's baffling how terrible this equipment is while the Comcast techs say they're the greatest thing since sliced bread. 

When I run a speed test on my computer connected via ethernet I get 500-800mb/second. However, there is always a significant 2-4 second lag before a website will start loading. Once the data transfer actually starts it's lightning fast, but the lag happen consistently. Something isn't right. I had Comcast rewire the feed line all the way from the tap to the modem. No change. 

Is this an issue that is known or experienced by others or am I on an island all alone on this one? I miss my reliable and amazing Comcast internet service. I just ordered an ax5400 router which arrives tomorrow. Last year I ran conduit from my house to my garage then pulled a cat6 cable from the Comcast modem all the way to the garage. I installed my Linksys ac1750 in the garage which improved the data transfer speeds and the cameras stopped disconnecting. However, the video would jump and lag frequently. Since Comcast has exhausted all of their options to remedy this situation I thought I'd order a high end router to replace my outdated ac1750 router. I'm hoping the ax5400 router will remedy this nonsense but I won't hold my breath. The game plan is to install the new TP-Link ax5400 router in the garage feeding the cameras then taking the Linksys router that was in the garage and placing it in my living room via cat6 from the modem to supplement the XB7's horrendous wifi performance. Comcast is also sending me a last generation pod to use. I plan to install the pod in the opposite side of my house where the Comcast modem is currently. If there's anything more I can do I sure can't think of it. I'm waiting in anticipation to see if this all gets me back on track. This has been an exhausting process. 

Has anyone experienced this kind of problem with the XFI complete? This has been the worst upgrade I've ever experienced.