Just changed to an iphone 6s. Tried to connect wireless, put in Reuter name and password but it won't connect. It shows a padlock next to router ??
The padlock next to the router means that the connection is secured (ie, a password is required to connect). Nine times out of ten, connection issues like this are from people using the wrong password. :-\
When you go to the Wireless settings, the router should show up in the list of available connections, Click that, then you should just need to enter the password to connect.
The padlock symbol simply indicates the router is password protected. I'm don't have any specific suggestions as to why you can't connect but it might be worth reading through Apple's help page on the subject: https://support.apple.com/en-gb/HT204051
[EDIT] Simon beat me to it but his point regarding your password is very valid.