There is no package (as of yet) for rt1900ac router by Synology...
DOWNLOAD PACKAGES FOR SYNOLOGY NAS
BITTORRENT SYNC 2.3.7 - 88F628X BITTORRENT SYNC 2.3.7 - ALPINE/ALPINE4K
BITTORRENT SYNC 2.3.7* - ARMADA370
BITTORRENT SYNC 2.3.7* - ARMADA375 BITTORRENT SYNC 2.3.7* - ARMADAXP
BITTORRENT SYNC 2.3.7* - ARMADA38X
BITTORRENT SYNC 2.3.7 - AVOTON
BITTORRENT SYNC 2.3.7 - BROMOLOW BITTORRENT SYNC 2.3.7 - CEDARVIEW
BITTORRENT SYNC 2.3.7 - EVANSPORT
BITTORRENT SYNC 2.3.7 - COMCERTO2K
BITTORRENT SYNC 2.3.7 - X86
BITTORRENT SYNC 2.3.7 - X64
BITTORRENT SYNC 2.3.7 - BRASWELL
BITTORRENT SYNC 2.3.7 - MONACO
* if you are using DSM5.2 or older for these platforms, please use next packages instead
BITTORRENT SYNC 2.3.7 - ARMADA370 (DSM5)
BITTORRENT SYNC 2.3.7 - ARMADA375 (DSM5) BITTORRENT SYNC 2.3.7 - ARMADAXP (DSM5)
BITTORRENT SYNC 2.3.7 - ARMADA38X (DSM5)
it's architecture is northstarplus:
RT1900ac - synology_northstarplus_rt1900ac
hope it gets uploaded soon