how many different serial cables do you have access to? i would say try a different serial cable (rs422)
https://www.8bitclassics.com/product/apple-8-pin-mini-din-serial-cable/there are more than one type of mac serial cable. see this description :
Unfortunately, there are two types of serial cables commonly used with classic Macs.
-"Modem" cables are wired straight through,
-"printer" cables are wired as cross-over/null-modem,
with TX and RX swapped (so that TX on one end goes to RX on the other end, and vice versa).
Connecting two Macs back-to-back for LocalTalk requires the cross-over style cable.
I'd guess that's your issue - most of the modern Mini DIN-8 cables I've seen are wired straight through
https://68kmla.org/bb/index.php?threads/apple-talk-trouble.40692/
you may be using a "cross-over" cable (ie: mac printer cable) thinking its a serial cable, theres literally no way to tell by looking at the cable.. well maybe possibly by checking some of the text on the side of the cable but yea its easy to mistake the two, and swapping to the proper cable type will instantly solve the problem
male to male din-8 rs422 straight through (not crossover)