So before you added frustum culling, the far-terrain was showing, and now it is not? I assume you draw your terrain in chunks, and some chunks are culled?
Are you using an existing engine, or writing your own?
It sounds like the far-plane of the culling frustum is set too close then.
Are you sure that your projection matrix and frustum cull use the same far-distance?
UPDATE based on your new illustration:
You can not afford to cull the orange marked chunks. You either:
Need to render them, and only cull behind camera, and out of the view cone.
Or add fog, so that terrain chunks that lie behind your far plane are not visible, because they are fully hidden by fog. In that case, you can keep culling them out as you do now.
Players can sometimes hate fog, so maybe it's best to go for option 1, which can be achieved by moving out your clipping far plane so that it is behind everything in your world.