poor upload speed

I just switched from Verizon to AT&T and purchased a Nighthawk MR1100 to replace the Verizon Jetpack. My upload speeds are terrible. The sales reps at the AT&T store are nice and want to help but I don't think it's in their skill set to fix my problem. I have one week left to fix this before I return it for a refund (minus a $45 restocking fee). Here's my setup:

The signal strength (RSRP) coming to my home is pretty steady at -92dBm. It's boosted by a SureCall signal booster model Flare 4G 3.0 sitting on my roof where the signal is the strongest. The amplifier is  positioned in my home office in the finished basement 100 feet from the antenna connected by coax. The booster's amplifier is positioned 5 feet away from my Nighthawk, which is positioned 3 feet from my PC (Windows 10). The PC picks up the Nighthawk's signal with an Asys USB-AC68 wireless adapter. My computer shows two available wifi networks, both from the Nighthawk. They have the same name, but one has a -5G suffix, so I expect that's from the tower's 5G signal. When I plug in the ethernet cable, I have three networks to choose from.

Download/upload speeds are fine for my purposes between 8 a.m.-12 p.m., but uploads are terrible every afternoon. So bad (less than 1mbps) that I cannot participate or conduct Zoom meetings (I do about 8 a week). Okla's speed readings (speedtest.net) for uploads between 8 a.m.-12 p.m. range from 1.4-9.8mbps. But in the afternoon then rarely go above 1. Often it's zero. The ethernet readings follow the same patterns and are not always stronger than the wireless. Even though I don't live in a very populated area, I thought it might be afternoon school sessions and business zoom meetings for those on lockdown, but the pattern holds through the weekend, i.e., good in the morning, poor in the afternoon.

Just for grins, I took readings with and without the booster's amplifier plugged in. Uploads were almost always significantly less. That confirmed the booster is not the problem. (Not that it should be, but I need to rule out all the variables.) Doing research on Netgear forums, I checked for my area's strongest channels---6 (4G) and 44 (5G)---and changed the setting in the Nighthawk from "auto" to 6 and 44, but it made no difference. I also changed the setting from Dual Band to 5GHz and found no difference in the upload. I changed them both back.

I never had such a fluctuation with Verizon's jetpacks. (I dumped them because their "unlimited data" plan choked me down to dialup speed after consuming 15GB of data. But until I reach 15GB, it was always fast and consistent.) Using my cell phone as a hotspot (instead of the Nighthawk) also generates an upload speed of 2 or less. 

I live ten miles outside of a small city in N. Michigan. There's not a lot of homes around here, and the AT&T tower is only 2 miles away. The Verizon sales person said I needed to log in and let my Nighthawk search for the best signal by clicking on "Reset Network Settings." But there is no such option in my dashboard or settings. I did confirm that my firmware was up to date.

So, there you have it. I'm stumped. So is AT&T. I don't want to have to return my Nighthawk, but I must get better upload speeds. Is there anything else I need to check or adjust? Could the problem be with my Asus USB receiver? I've attached an image of the Excel sheet with all my readings relative to the three networks and the times of the the day. 

Thank you in advance.