It has something to do with sentry mode. Turn sentry mode off. Leave the cybertruck for 10 minutes. The problem should go away. You can then turn sentry mode back on.
The flat mount from TRIO
https://www.trioflatmount.com/shop/p/gen3speedmount
mounts very easily on top of the rack from Cybertruck
https://cybertruckco.com/product/tesla-cybertruck-crossbar-spare-tire-mounting-kit/
I was able to achieve 300 Mbps or more sustained on the interstate (70+ MPH).
This happened to me as well a few miles after accepting delivery. Pushing dash-wards on the plastic part that the squircle is adjacent to seems to fix the issue.