I've just tried a portable/pocket wifi router and set up SSID and Password.  Immediately afterward I could no longer access the interface (Win 10, btw).  Tried all the usual tricks including the ipconfig and netsh ones, as well as primary WAN/LAN router, but nothing.

I can access the primary router interface no problem at all.

Pushed reset button, and access was restored via usual IP address ... most weird

There are quite a few similar shout-outs for help on the 'net.

Most weird indeed.

My problem is, that I can not for some bizarre reason, get it to work on my computer (and ONLY my DELL XPS 13). It has worked, uninterrupted, smoothly and as expected on every other device I own, just not this darned computer. 

Nope, I use AVG, and tried both disabled and "allowing" the SSID.  However, I think this may be the short answer in my regard -

... this is by design and is expected behaviour. Your router's IP address has likely changed based on the DHCP from your modem. By changing from router to AP mode you're basically asking it only send and receive WIFI but lose all of the routing functions. Consider all of the things that your router usually does: dhcp, firewall, network routing, etc. This all gets shut off and relies on another device to provide those services.... "

Now, if I can find the AP router's IP address, and set it permanently in the primary router .... maybe ... ??

yitz - I think you are right.  I have another USB dongle style wifi router (Chinese brand) that allows me to access the interface all the time by simply typing the IP address, and another box-like AP (LevelOne brand) that also allows one to also always access its interface regardless of settings.  So, 2 out of three ain't bad

At least the errant one is fairly secure and requires a person to physically re-set it to gain access.

SATTV: Try using an Ethernet cable rather than wifi, some devices lock you out of the configure via wifi once they are set up, there may be a box to tick or untick when you do the initial setup.
John

That. :-)
No surprise if the router blocks admin access via wifi.

Also try UNINSTALLING kaspersky . Disabling doesnt allways work as expected
Ive seen many weird issues with 3rd party firewalls blocking things unexpectedly. Only fix was uninstall .

Otherwise , add 192.168.1.254 as an allowed site/exception in the Kasp firewall .

Problem solved. Well, sort of...

I got tired of trying everything and gave up. Got a new modem and everything seems to work fine. It still bugs me that the problem was isolated to my one device (XPS13) and the old modem. Right now, I'm just glad I can change my wifi name to something stupid.