Sending ipv6 Router Advertisments to wan interface about ULA prefix

I operate two networks here, my "home net" and my "desk net". My LEDE router connects these two, the "home net" being behind the wan port and the "desk net" behind the lan ports. The (non-LEDE) home net router which also provides access to the internet and the LEDE router acquires a global ipv6 prefix via PD from the home net router.

Now to allow local communication independent of the global prefix provided by my ISP I configured ULA addresses in both networks ("home net": fdb0:5279:7365:0::/64 and "desk net": fdb0:5279:7365:100::/64).

The missing part in the puzzle now is that the clients in the home net learn that they can access the desk net via my LEDE router. (The other direction is no problem as the LEDE router serves as default gateway.)

To accomplish that the (IMHO) best option would be if the LEDE router sent router advertisements (RA) into the home net, announcing itself as router to the desk net ULA prefix.

I wonder how I could do that. Normally I would have picked radvd for this job, but this package doesn't exist any more in LEDE (or OpenWRT) and odhcpd is supposed to fulfill its tasks. I failed to find in the docs if (and how) the latter can send RAs to the wan interface. Can somebody give an advice here?

The obvious alternative is to teach the home net router about the desk net ULA prefix but that's less pretty because it hardcodes redundant data and so doesn't benefit from the nice automatic stuff built into ipv6.

Thanks in advance