I have not found anything yet. The two most promising possibilities (imo) would be OpenWRT or DD-WRT. There's a thread for each:
https://forum.openwrt.org/t/anyone-working-on-netgear-rbr850/
https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=326993&sid=618c39521d6047ce3a7cb9c17fadd408
Both...