From the Dex page:
The Samsung DeX service is currently available on the Galaxy S8, Galaxy S8+, Galaxy S8 Active, Galaxy Note8, Galaxy S9, Galaxy S9+, Galaxy Note9, Galaxy S10, Galaxy S10 5G, Galaxy S10+, Galaxy S10e, Galaxy Tab S4, Galaxy Tab S5e, Galaxy Note10, Galaxy Note10+, Galaxy Note10 5G, Galaxy Note10+ 5G, and Galaxy Tab S6.
? The features that are available may vary slightly by model. We are planning to provide software updates to expand the availability of the service.