I have lost my remote, and I don’t know which one that is compatible to purchase. I have the Express+ Model #3910X which it was purchased in 2019
The 3910 only works with the basic IR remote. You can get an official Roku remote for $9.99:
https://www.roku.com/products/accessories/roku-simple-remote
There are many universal remotes that will work.
Be careful as there is also a Roku simple remote for TV's. It may not work with a standalone Roku device. The correct model number is RC909.
Unless you need the composite video output option, it may be time to upgrade to a newer Roku that will be much faster and have more channel storage memory. The 3910 was released in October 2017.
Per the chart on this page, Roku Voice Remotes can also be used to control the 3910.