FFF,
It's been a while since I worked in an area with advisory routes, but IIRC....
In the open FIR the advisory route is Class F and you'll be provided with a radar advisory service if the route lies within a radar advisory service area. RAS = IFR, though it may be possible to negotiate a RIS on an advisory route if you can't accept an IFR clearance. There's also the possibility of flying on an ADR with no radar service available, but we won't go down that road here.
Where the advisory route enters CAS (A or D) it ceases to exist as an advisory route and becomes just another ATS route within CAS, with the appropriate rules for flight in that type of airspace applying.
So if you're flying on an ADR and it enters Class A CAS, you need to be appropriately qualified to fly in Class A after the point at which the route crosses the CAS boundary. Same applies for Class D.
With an IMC rating only you could accept an IFR clearance on an ADR for the part that lies in the open FIR and if it penetrates Class D, but could not continue on the route from the point it enters Class A airspace.
Does that help?
WF.