I'm not sure the model. I got it from my sister. It's in a green box and just has the name roku, no number. The serial number is 2MD5AW001483. I have tried to follow that link but it says it doesn't exist anymore 😞
Its not really worth buying a remote for, and there isnt a currently available remote for the 3500 (current remotes are 3600 and later).
Furthermore, Roku.com doesnt directly offer replacement remotes (or other accessories) in the UK - they use an external company who's ebay site doesnt exist. Youd have to buy it elsewhere (Curry's?) - you can look on ebay etc, but again, its not worth it - thats a 7 year old model, and you can get a whole new Roku (including a remote) for £30-50 - I recommend the Express 4K.