it is ok

for special arrangement like working from home not to go in the offer letter.
Is it in the offer letter that you will get shares in five year, and how much? If not, you can't count on it.
It is still good to ask for a small portion of shares. What you hold is key position, and it is reasonable to ask for it. I am surprised that they only offer that after 5 years. Don't sell yourself short. However, this shouldn't be a deciding factor. You should still go, but you should maximize your return too.