Hard to tell what the problem is without looking at the RTP traffic. The
log looks fine. The delay you mentioned could indicate that it might be
the kernel module that's misbehaving. Perhaps you can try the same thing
again, but without the iptables rules installed (and/or without the
kernel module loaded), and see if that makes a difference.

That's curious, perhaps something went wrong when compiling the kernel

module then? I have to admit that I have zero experience with rtpengine
on Red Hat myself.