Are you running the stock firmware?
Awhile back I flashed grblHAL, built with the grblHAL web builder, to my SLB on my LongMill Mk1. Everything was working fine until I needed to use my laser. I couldn’t get it working and wanted to proceed with my project so I flashed it back to sienciHAL and my laser worked again.
I don’t know if I missed something in the web builder configuration or the settings somewhere in gSender but I got it working pretty quickly when I switched back.
My laser is 3rd party with it’s own power source and I just need a PWM signal to control the power so I think the setup is probably the same as yours.
Not sure if any of this relates to your problem but I thought it was worth mentioning.