Delete (or make a backup of): Outward\BepInEx\, Outward\Mods\, Outward\winhttp.dll and Outward\doorstop_config.ini.
If you have previous mod installations it is recommended to remove all existing mods and mod loaders before you start using Thunderstore/r2modman.
Make sure you are on the mono branch of Outward (see above).
r2modman is a mod manager which is integrated with Thunderstore, and supports Outward along with other games. Thunderstore is a mod database which supports Outward.
On Epic Store: you are on mono by default and dont need to do anything.
If you see the "radio buttons", make sure to click the radio button titled Version X.X.X:mono (where "X.X.X" is the highest version number you can see) to update to the correct version. Select the drop down and select default-mono. From there, go to Manage Installation > Configure.
On GoG Galaxy: From the Outward screen (where the play button is), click on the Options button to the right of the play button.
On Steam: Right-click Outward in your Steam library, and choose: Properties > Betas > "default-mono - Public default branch (mono)", there is no access code.
In order to use mods, you need to be on a Mono branch of Outward.