I've noticed something I think is very strange behavior for a local network.  I have 2 X60 units, one in the lounge room (main unit), it has a ZTE (NBN) modem plugged into one of the ports, the second port has a Synology 1019+ NAS attached which has a static IP.

The second X60 is in my home office at the other end of the house, with my work laptop connected via Ethernet to one of the ports.

Normally all this works works fine, I can browse locally to the NAS, but recently a tree fell and took out our NBN lead cable to the house so we had no internet for a few days.  When this happened I couldn't get to the NAS.  TP-Link app was showing it as an offline device, plugged into either of the main units ports, but when I take the NAS and plug it into the second X60 in the office it shows back up on the network.  I figured the ports on the main unit had fried.

Then the NBN tech came and reconnected the line, since the internet came back on via the modem plugged into the main unit the ports were clearly working so I tried plugging the NAS back into the main unit and it works again.  I can get to it locally via the main unit.  A few weeks later our internet got knocked out again, and once again could no longer get to the NAS unless I take it and plug it into the second Deco.

TLDR - When my main unit X60 has no internet connection I cannot locally get to my NAS plugged into that main unit, but I can access it if I plug the NAS into my secondary X60.  Is this just some way mesh networks function?