Just quickly concentrating on the brakes here and not the running lights because I have to run.
Lets do the easy stuff first
Are you positive that your brake controller is working?
Have you checked the trailer's fuse for the brakes? Checked fuses in truck?
Assuming those things are fine, that seven pin circuit tester will help to check the wiring plug connection. You can also use a 12volt test light or your voltmeter and save a few bucks. Its a good start.
If none of that, than there are other steps. Al-KO manual
for your reference.