I have a B525s-23a, it failed to update using local update. I eventually tried the update on page 1.
Undo 3 screws, separate the case.
Download the software, follow the instructions, which all run without any knowledge needed.
2 old USB mini cables were joined by cutting off the mini connectors, then solder the red, black, white and green to each other. Or get one from ebay.
Plug the modem into the power supply, with the power supply switched off, hold a screwdriver on the 2 terminals (left pair), switch the power source on. No LEDs will light up.
Run the USB program and the LED will come on.
Complete the update.
When you switch on it will be in Polish, language is top right.
It may retain your previous password.
Now works with a telephone in the RJ11 socket. It is very straightforward.