Hi Odai,
"I think you always need to be in two-way radio contact with the ATSU if inside the ATZ, and if the ATSU is an ATC unit you need to obey all instructions given by the ATCO."
Incorrect.
Glasgow (ATC) and Cumbernauld (ATZ with A/G) allow non-radio traffic (see AIP).
Of course, it is many moons since anyone even dared ask to go into Glasgow non-radio!
So, two-way contact - but could be by light signal or (as has happened with one of our chaps) relay from another aircraft in formation.
ps. Cumbernauld, as far as I know, now doesn't have an Aldis lamp. H&S said they couldn't keep the battery in the tower. So they would have had to carry it up and down the stairs. Folks said: if we go down the stairs, we can just use the radio in the fire truck!