Can't find the missing piece to the puzzle

Another apparrent Comcast fiasco to report, I'm afraid. Laptop WIFI was doing 200+ consistently with Blast 200 Mbps and an XB3 Modem. Upgraded to 400Mbps and was instructed by Comcast to re-boot the modem to "install" the upgrade.  The only outcome of the re-boot was my WIFI speed instantly dropped to 7 Mbps on both my laptop and cellphone WIFI.  Comcast vigorously denied any cause/effect attributable to them. Next, told me I needed XB6. Next day, Tech brought the modem. My ethernet connection registered 400+ but WIFI still < 100 Mbps on the best of Speedtest. However, Tech showed me that he was getting a strong WIFI signal on HIS phone, so declared he has done his job, my WIFI card must be dead, my problem was not his problem, and left. Laptop being out of warranty by 6 weeks, Dell Support was no help. However, bought the laptop at Costco, so their Tech Support was available. Two separate Techs put in better than decent effort, lots of diagnostics which found nothing, helped me update all available Drivers including the latest WIFI,  and some blind shots in the Dark, but basically no significant change in my abysmal WIFI speed. before they gave up being out of ideas. This has now stumped a handful of people, some of whom actually appeared to have some tech skills. Anyone been down this road and/or have any idea what is going on, and how to fix it?  Any help at all would be appreciated. I am dead in the water and now tethered to an Ethernet cable draped along the wall and running under the door.