The WRT1900ACS is part of the wrt_ac_series, with 4 detachable antennas, and a dual core processor. It is a dualband, 802.11ac wave 1 (80MHz) capable device.
Both revisions of the device are hardware identical, with changes made to the way wifi power tables are loaded into the device due to moves from Linksys in response to FCC changes.
If the firmware has become unresponsive and won't boot, chances are you can recover it easily due to the Dual Firmware partition layout, by switching to the alternate firmware.
For this reason, it is recommended to always keep at least one known good version of the firmware on either partition.
Switch router from primary to alternate partition or vice versa using any of the 4 below methods: