Could be the broken I/O board issue (if i recall - don’t quote me) discussed in the other NT505 thread. I actually thought yesterday I might be dealing with same thing.
Using wifi extender in second system far away from router, and then cat 7 from extender to NT505. Had worked mostly flawlessly for couple years but lately wasn’t connecting as well.
Used as opportunity to finally upgrade firmware (done via micro USB cable from PC directly - network not needed) to go from firmware version 1.21 to 1.30, current as of November 2021.
Then took the unit downstairs and directly connected to the router via same cat 7, and it worked, so I knew it must be my wifi extender.
Anyway, I had another extender, paired/linked that to the router, and now working again upstairs. But it feels a bit tenuous....... Always takes a bit to lock on and I’m never quite sure - have taken to leaving it on and hopefully connected all the time, see how it goes.
BTW I use the Lumin network app, not the Teac network app - as for me it is preferable. But all that said, I’ve gotten 3 years of great use out of the dac and still like it very much