Lots of engineering students would pay you if you help them with their assignments. However, this can't be very ethical...

A better option can be, if you are able to build a system simulator in your area of expertise, lots of companies would pay you to use it. This is an example of big system simulator, but this is open source. I don't know what your area of expertise is but let's say instead of telecommunications, it's aerospace: if you are able to model any plane complex system, I guess universities and companies will be very interested. You may need to do a bit of research first and then create a product and try to sell it.

If you are able to build a system simulator in your area of expertise, lots of companies would pay you to use it. This is an example of big system simulator, but this is open source. I don't know what your area of expertise is but let's say instead of telecommunications, it's aerospace: if you are able to model any plane complex system, I guess universities and companies will be very interested. You may need to do a bit of research first and then create a product and try to sell it.