You have tried the SonicWALL Mobile Connect on W10 and it connects well, that basically means the VPN gateway side should be working as expected. You have tried different RDPs and no one works as expected though the VPN status is connected. Is my understanding correct?
If yes, you haven't tried any protocols other than RDP. Are you able to PING or access your internal website once VPN is connected from Kindle Fire?
I opt out so you can seek more helps from other experts.