@Alexandre. Hi Alexander. Yours is much the better solution but you have to configure the NAS as you say BEFORE trying to switch to a mesh network. Otherwise you have to reinstate the old router to connect to the NAS to make the changes. One other point. By "factory reset" I mean the reset achieved by pushing a paper clip into the small hole at the back of the box. This procedure deletes no files or data, only a few settings which are easily restored. I did this yesterday. All my files (music, photos, text, videos, etc) are unchanged.

  @Soluna 

To be clear: insert a paperclip into the hole at the back, press gently for four seconds until you hear a beep. Remove paperclip immediately. This will reset the DSM but delete no data. NAS will reboot. Use admin and enter no password to access and restore your settings. This is a Factory Reset. As Alexandre warns, it is possible to delete all data if you continue to hold the button after the beep for about 5 seconds. This is a Full System Reset. One way to ensure your data is safe is of course to remove the hard drive(s) before you do anything.