Yeah, you can start by resetting the NVRAM of the router, (30-30-30 reset) then get a flash chip clip, read the data off the router flash using a raspberry pi, and compare it to the firmware binary from the router manufacturers website.
I am an experienced SWE and this is not something I can do without setting aside a day or two to investigate all the tools and purchase an RPi.
Oh, you sold a piece of shit insecure WiFi lightbulb that's mining bitcoin, here's a fine for every penny you made.