Your router should give a field for public port as well as private port as you’ll need to input both fields. The range isn’t the whole public to private ports, but should be just a single number. For example if you’re trying to port 8887 use the range from 8887 to 8887.
If you can it is also recommended that you assign a dhcp reservation for the Tablo ip address through your router.
According to the link below, if this is what your UI for the modem/router looks like then you cannot forward an External Port to a different Internal Port. This is an inherent limitation of your router.
My recommendation is your put your modem/router in bridge mode and use it as a standalone modem alone. Then get a new router. Even cheap $20 routers these days have proper port forwarding functions.