I assume you are sure that you have 850MHz service in your area?

If so, try setting the phone so that it just uses 850MHz

gdfswrite:000000AC0B

And then see if you get a signal - I would suspect you won't, since the only phones I ever heard of this working on were some early production versions of the k790. If it doesn't work, then you can restore the original bandlock using

gdfswrite:000000AC09

I would be interested to hear the result, anyway..