I have a ZyXEL P-873HNUP-51B it's special because it offers bonded DSL on two phone lines.
Is there a similar ZyXEL for which OpenWRT firmware may be available?
ZyXEL does not publish firmware for this model (it is carrier controlled) and mine was hacked.
https://www.zyxel.com/us/en/support/DownloadLandingSR.shtml?c=us&l=en&kbid=P873HNUP&md=P873HNUP