Netgear XR300

Overview
"U12H332T78" is in the header of the firmware file that is available for download.

Broadcom chips are just a guess, for now. This device is presumed to use BCM4360 + BCM4331 radios like the Netgear R6700v3 / Netgear R6400 v2 since no MU-MIMO or 256-QAM support is claimed in any currently available PR.

Links of Interest
Datasheet

Flashing Tomato
Support added

Flashing ExpressVPN
Compatible with Netgear Nighthawk R6700v3 firmware

Serial Pinouts
This router had a 4 pin header already soldered onto it and the pins were clearly labeled on the printed circuit board's silkscreen.

1. +V 2. Ground 3. Transmit 4. Receive The Baud rate is 115,200.