Question

I am his neighbor. He did not help me. (Join into complex sentence)

Answer 1

He did not help me though I was his neighbour.

Answer 2

He did not help me though I am his neighbor.

You can join two simple sentences to make a compound sentence.

In this case, you have to join the two independent clauses, and you can use comma between independent clauses to join the sentences.

But that can be applicable for the coordinating conjunction only.

Normally, you can join the sentence by using noun and phrases.