Guest SSID Separation in AP Mode?

The way I understand it, by putting the N66U in Access Point mode, you are essentially putting it in bridge mode and all SSIDs are in the same broadcast domain (and losing other modes you lose from router mode).

That said, are there any "tricks" or workarounds for the N66u that will allow me to have separation for a guest network (SSID)?

I currently have :

[modem]-[router]-[N66U AP mode]

I am aware that using the N66U as my primary router would alleviate this issue. I am hoping to find out if my current setup is feasible at all for my needs.

Humbly asking,

First time poster, long time reader

P.S. Running 3.0.0.4.374.35_4 (Merlin build)