For the record, the ASUS OnHub is now supported in master, and snapshot images are being generated.
The TP-Link version has the best ToH documentation, but ASUS is very similar: https://openwrt.org/inbox/toh/google/onhub_tp-link_tgr1900
And there's a more active forum thread at Onhub TP-LINK TGR1900 future support? for TP-Link (and mostly covers ASUS too).