I installed and synced this mod to a current play-through and it does not extend the pipe length. Does this only work with a new start? I have confirmed it is enabled on the mod list for the save.

Nevermind. I did not realize this had it's own research tech.

It doesn't work for me, the length is set to 50 in the settings, but the underground pipes are still short, what could be the reason?

The mod added a separate entity with the longer distance, the vanilla underground was unchanged. So you need to first research the tech and then craft the longer pipes, which should be found right next to the normal undergrounds in the crafting menu.

However I also updated the mod so you can now choose wether you want the longer underground as a separate entity or if you just want to modify the normal underground's length in the mod's settings.