Module not found when I do a modprobe

If the module .ko file is really under /lib/modules/4.3.3-5-default/extra/ and 4.3.3-5-default is indeed your current kernel version, then the problem may simply be that you need to run depmod to re-create the module dependency list. Run:

sudo depmod

and try again to modprobe the module.

My solution is unique, but in my system I rebuilt the kernel and took out a module. I installed the vendors module and could not get the system to boot using the vendors module. In my case I forgot to move all the /lib/modules info, so modules.builtin still had the module, that I was upgrading. Manually removing the module from modules.builtin and doing the depmod -a fixed my problem.