I struggled to connect to the USB port to fix some code that was not working on the MKRWAN.
All the online advice was to push the reset button twice to enter a boot loader mode, but this would not work for me.
The running program used the serial port extensively, and I powered the device via the USB port.
After a lot of struggling, I powered the MKRWAN from an external supply, then pressed the reset button twice, and then only did the yellow LED blink.
Now I could plug in the USB connector and connect to the device's programming port.
I am writing this to help someone who finds themselves in a similarly desperate situation that I found myself in.