Any thoughts on a dramatic decrease in speed shortly after implementing USB tethering on an R7000?

The situation - I'm trying to get rid of AT&T DSL and replace it with USB tethering or a hotspot.

Router - R7000 with the latest DD-WRT installed

Internet source - Goggle Pixel 3a with a Visible unlimited data plan

Test network - the R7000 with the Pixel connected by a USB cable and a laptop connected with  an ethernet cable.

After some Googling and fiddling around, I arrived at the eureka moment.  The laptop had internet access, and about 4 times faster than my DSL.  I let the test network run for about 3 hours, including an hour of music videos.  With that success, I decided to disconnect the Linksys (DSL connection) from my network and connect the R7000.

For a few minutes it worked well.  I decided to install a WINE upgrade to the laptop (now WFi connected).  After a few minutes the speed dropped abruptly, and dramatically - well below DSL speed.  And not just WiFi, my desktop with a wired connection experienced it too.  Eventually, the download timed out.  I switched back to DSL, and it was all back to normal.

Any ideas about the cause and fix for the speed drop would be appreciated!  The R7000 did feel a bit warm.