There are many rights you can't waive. For instance, you can't sell yourself into slavery--everyone except for hardcore libertarians is fine with that one.
And you can't waive your right to remain silent permanently. You can reassert that right at any time.
The right isn't really to "remain silent", it's the right to avoid self incrimination. If you have immunity, you still have that right, you just can't incriminate yourself.
And you can't waive your right to remain silent permanently. You can reassert that right at any time.