Rawtherapee plug-in not visible in GIMP snap preferences

I am using GIMP 2.10.6 and RT 5.4 on Ubuntu 16.04.
GIMP is a snap, and RT was installed from Software center.

Although I can see file-rawtherapee in the gimp/snap plug-in folders, it is not appearing in the GIMP preferences dialogue. See scheenshot below.

So I am unable to open a .NEF file from inside GIMP via RT.

How do I enable the RT plug-in in the GIMP snap?