How to make sure the security of wifi router devices?

I need to give some advice to some wifi router equipment manufacturers on security.

Their devices will be sold to public places such as Cafe, they need to do their best in security to protect the user who connects to their devices.

I've searched for some technology advice, but I couldn't find an answer.

The only thing I know is the router should forbid the users who connect, to communicate to each other. To prevent hacker in the same network attacks other devices.