I use myself R7800 as main main router, and WNDR3700v2 (or WRT3200ACM) as a dumb AP without problems.
(I have a have a dumb switch connecting the fixed cabling between various rooms.)
I use my own simple dumb AP recipe, which only requires minimal config changes. No changes to router's switch config needed, if you just leave WAN port unutilised. The advice pretty much matches
But the R7800 + WNDR3700 combo works quite ok for me. And yes, naturally I am running my own builds in the routers.