So this is a repeat of what Bolt owners went through, and a big +1 to HTC phones for having the ability to do this.
Dial ##778#, call
password is 000000
menu > commit
phone will reboot and you should get 3G back
This changes your network from eHRPD back to EVDO Rev.A. For LTE phone owners, eHRPD causes 3G to die (most of the time anyway) when the LTE network has problems. Get back on normal EVDO and you're good to go.
Once the network is back up, you'll want to repeat the steps to turn eHRPD back on, as the 4G/3G handoff is VERY slow on EVDO.