ok, I know I am a little late to this party... but here is my take.
I think it has been decided you have a Dish 300 system. (Does the lnb you have now, look like 1 eye, or 2 sitting side by side?) Probably just 1.
If that is the case then you need to return the DP lnb you have, cause as it was already mentioned... the DP lnb will not work with your legacy receiver (the 4900).
You need a "legacy dual lnb".
You need to run a second line from the dish to the new receiver.
I know you have the splitter in the line now, and it has been there... but because it is on the power-passing port, it is acting as a barrel connector. The receivers have to be able to send a specific voltage or tone depending on which channel you are trying to watch, to the dish. So each receiver needs its own direct connection to the dish.
And whatever idiot at Dish tech support told you that, is wrong. And the local guy may not have quite understood what was going on... because he did sell you a dual lnb, he just sold you the wrong one.
Welcome to the site!!
and if you stick around and continue to ask questions, you will learn to find... when it comes down to the information you are told by a tech support rep and us.... we are right

A lot of us on here have been doing this work for a living for a long time, and know what we are doing.
Good luck, let us know how it goes.