@Rekka in what ways that it doesn't work? It's currently working well for my Android phone and Kindle fire. Maybe recheck if the option is on? Go to Reader settings, make sure you choose the Display advanced settings option, scroll down near the very end to see if Compressed or original image has been chosen.
Clearing cache and/or site data also seems to fix a lot of problems so try that as well
Someone said this to me "Mangadex disabled data saver while they are rolling out md@home. Keep it enabled and when they bring back server side it will work again" regardless of md@home turned on/off the compressed image is not working but the original is loaded instead
Edit: data saver is working right now.