Were your M9's purchased together? If so they should all be the same hardware version.

But your satellite is running firmware made for V1 M9 Plus while your Main is running firmware for V2+.

You can update firmware manually at tplinkdeco.net and by downloading from here (select the right hardware version first) https://www.tp-link.com/uk/support/download/deco-m9-plus/v1/#Firmware

If firmware changes don't work maybe just try removing it from your network and adding it in again. I'm no expert but a lot of posts seem to go without help on this forum so hope this gets you somewhere

  @ConcernedUser 

they were purchased together.

Thanks for this. Am I better making them all equal the same firmware for v2+ ?

  @ObiWanKenobi90 It depends on what version your M9 Plus are. It should say it at the bottom of the unit if you're not sure. You need the right firmware for your device and hopefully that will fix it