This is for Posterity: Googling issues with Apple iOS 9 Mikrotik and L2TP VPN lead me to this Post. I had many troubles and finally got it to work. I will post my commands below to hopefully save others some headache. :D
this assumes some default configuration on the router: local subnet 192.168.1.0/24 router address 192.168.1.1 dhcp pool 192.168.1.100 - 192.168.1.200