If you are still working on this - and assuming you are talking about an RDP client, not server - I have started picking apart the open source ios/andriod freeRDP (the only clients that I know of that are completely free use, meaning you can make app you can distribute on the app store, which you cant with most open source clients)
The implementations are in java and objective C, which can be integrated with flutter through several methods (which I am currently researching)
I was curious if you had made any headway on this because I have also hit a bit of a dead end with trying to figure the implementation of RDP with flutter