I am in a similar situation. My GL-AR300M-Ext ran out of memory and became unresponsive. Did a full reset and am now stuck with not being able to connect to the setup SSID.
Neither goodlife, nor PL20180515 is valid.

I am on the road without a laptop to hook up and do command line wizardry. Anyone knows the default password of the latest firmware this device could have auto-upgraded itself to?