When I joined Spark a while ago, the trusty RT-AC66U running Merlin would not work, although it had been fine on Bigpipe.  I searched here and on Google but either found instructions that didn't work or threads with contradictory information.

 

Anyway, in the hope that others won't have the same issue, use the PBTech instructions for the RT-AC5300, etc.  These do work for the AC66U as well.

 

https://www.pbtech.co.nz/ASUSISPSettings#SparkRTAC

 

I did change back to the Asus firmware as Merlin no longer supports the AC66U.