I don’t think anyone can understand fully your current setup from your description of it.
If you could post a simple network diagram (even a picture of a hand drawn one would do) and a copy of your current configuration on the MIkrotik, someone may be able to help you.
To post the configuration follow this:
http://forum.mikrotik.com/t/forum-rules/173010/1
Is the SXT the only Mikrotik device involved?
Is there a switch (managed or unmanaged) involved?
Which make/model is the device you call “router”?