Sorry, the Mitigate MTU option is only relevant to the Vero and your using a RPi. If your not seeing the credentials getting added with debug logging then there is probably still something wrong with your passwords.xml even if it isn’t throwing an error. I would suggest to either take a very hard look at the file (you can use this how-to for reference on what that file is doing), ditch your current one on this machine and replace it either with making a new network path in Kodi or just copying that file from your working machine (it doesn’t matter if it is on another platform), or else you would have to give us a pastebin of the exact file that had only the credentials replaced and no other edits so we could have a shot at spotting the issue.
Generally speaking it doesn’t take much of a mistake to break an XML file in Kodi. For example I was having an issue with a keymap I was working on the other day and it took me a half hour before I figured out it wasn’t working because I had accidently inserted two spaces instead of one between a key id and its modifier, and that was enough to make it not work.