Hi,
First of all, check if you have updated your RUT240 to the latest firmware available.
Then, go to the same Mobile page in RUT240 WebUI and next to APN there must be a dropdown menu, you just have to select the Custom one.
Once that is done - you should be able to write down your own APN, in this case, you must use wireless.dish.com
This should be enough to make SIM data work.
EB.