When you have servers in your network that needs to be accessed by internal users using the external address, you will need to setup hairpin NAT also known as NAT loopback on your router.
In Mikrotik fashion, this isn't configured by default.

To configure hairpin NAT on your Mikrotik router, do the following:

1. Go to IP > Firewall using the WebGUI or Winbox
2. Go the tab NAT
3. Create a new rule containing the following
   
   • Src. Address = your network ip/subnet, for example 192.168.0.0/24
   • Dst. Address = your network ip/subnet
   And yes, both entries should contain the same value
   Then go the Action tab and set Action to masquerade
4. Now make sure your new rule is at the most top of the NAT list, you want to make sure all traffic will hit this rule as much as possible

Now you should be able to reach any server in your network using the external address!