Hello,
is the WiFi module still powered in the same way as in the case where you were able to connect by Firefox?
If you can connect there with Firefox then it is possible to connect there also with PC software as you know from USB interface (you can enter the IP there with CTRL+W).
But definitively if you are connected to the Wifi Link network directly, you should be always able to connect to
http://192.168.4.1 even that the Spirit unit is not connected.
Sometimes Phones are switching between networks on its own, maybe this happened.