Hi, and welcome to the SNB forum!
I don't know if you have resolved your issue yet, but here is some information about msftncsi:
(nsci = Network Connectivity Status Indicator)
msftncsi is used by the router web-server uhttpd (/usr/sbin/uhttpd).
At times it tries to download
http://www.msftncsi.com/ncsi.txt
This is also a common practice by all Microsoft products, e.g.
Skype, Office 365, Outlook, Microsoft Feedback Hub, Microsoft Store, Cortana, OneDrive, Defender,
Xbox Console Companion app (and all games there) - but not Windows Update.
This is what determines the state of the windows system tray network indicator icon.
It's also used by 3rd party applications such as
Adobe, Google, Spotify, Drobox, and gaming platforms.
Microsoft stores logs of all accesses to
www.msftncsi.com!
These logs contain the time of each access and the IP address recorded for that access.
PS
Similar requests/downloads are done by other manufacturers as well, eg:
Apple devices request :
http://captive.apple.com/hotspot-detect.html
Google devices request:
http://connectivitycheck.gstatic.com/generate_204