TrustZone is a CPU mode, hence it is not fully isolated from normal CPU operation. The CPU chooses to enter it and the current CPU state gets saved/restored. It contains the highest exception level, so it is able to access all memory. It does not usually have networking because that would invite complexity, but there is nothing to stop a vendor from putting a full network stack in there and assigning a network peripheral. Typically, it would rely on the main OS to send and receive packets.
Trustzone is a secure execution environment, mostly isolated from normal CPU operation. Wasn't it so that it cannot even access main memory???
ME is really more privileged than the CPU?
I have not heard about Trustzone doing networking. But ME can supposedly do even WLAN while the CPU is not running.
Disclaimer: I am not a hands-on expert at that level, more like an armchair pilot...