The 3900 is one of the Roku models Disney Plus says it supports.
Disney Plus is now live and available to stream in the US, Canada, the Netherlands, Austria, Germany, Italy, Spain, Switzerland, France, India, Belgium, Denmark, Iceland, Finland, Norway, Portugal, Sweden, United Kingdom, Ireland, Isle of Man, Monaco, Wallis and Futuna, French West Indies, French Guiana, New Caledonia, Japan, Indonesia, Latin America, Réunion, Mayotte, and Mauritius. The service is set to expand to Singapore, Hong Kong, and South Korea later in 2021. (Note no mention of Thailand.)
The above is per this Business Insider article, 12 May 2021 (check article for updates):
PLEASE NOTE: In addition to enabling each country for web streaming (via a web browser on a computer), Disney must also release a version of their channel app compatible with each streaming platform such as Roku that is authorized for use in each country. This may lag behind release for web streaming. I can find no reference listing which countries have Disney Plus apps for Roku.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X