There are two ways round this:
1) (the easiest) - see if your router supports "NAT Loopback" (may go by a different name depending on brand of router), if it doesn't - upgrade to one that does. This will immediately make your service work.
2) (The hardest) - This is really simply a workaround for when your router doesn't support nat loopback. You need to do something called "Split Brain DNS". This is where you have a different DNS server that your router forwards queries to instead of the main internet/ISP DNS one.
For example, if you installed a DNS server on your raspberry pi (bind is really simple to use in basic configuration), you could then set it to forward all queries to your DNS server of choice, but, you can host a file for the devita.co domain with the local ip.
This basically means that devices on your network will refer to this DNS server and get the internal IPs whilst machines out of the network will get the regular DNS and external IPs.
This is the official way it is done in some big networks, but, I personally do not like it!
... There are good reasons for both though, and even if the router does support NAT loopback, there are genuine reasons for using Split Brain DNS.
For example, if you have the local IP, you will make a direct connection - if you have the external IP and use natloopback, it will go via the router and as in its name, loopback and duplicate the data over the network. So, in large networks, there are genuine reasons for using this even if your router supports NAT Loopback... but, NAT loopback is so much easier!