If I'm understanding you correctly, it seems you removed your extension in one piece, and then attempted to reinstall it the same way. The Remington extensions have a two-piece construction to minimize the transition from the magazine tube to the extension tube. Because of this, when the extension is off the gun, the extension tube can be threaded deep enough into the coupling nut to prevent the coupling from being tightened down on the barrel guide ring.
To properly reinstall a Remington extension, take the extension apart, and install the coupling by itself first, tightening it against the barrel guide ring. Then, insert the magazine spring (and follower if needed), and thread the extension tube into the coupling, taking care not to pinch a spring coil or leave a gap inside the coupling.
Here's a good video on proper removal and reinstallation of a Remington extension.