Modbus “broadcasts” (using id=0) have been considered before
In essence, you can make broadcasts to id=0 from the binding but may get warnings. (no response expected, of course)
I don’t think the changes needed to handle broadcasts warning-free have been put it place yet, but they would certainly stop you doing what you’re trying to do, use id=0 as ordinary read poll.
I see no reason to break the rules of Modbus protocol for everyone else in order to accommodate one person.
Put it another way, other people have made this device work with no such fiddling. I believe that has been done via connection to the RS-485 serial instead, though.
If your device modbus id=0 and cannot be changed, then it would be broken and useless. So we can figure out from that, that maybe the problem is that you haven’t found out how to change it…
I think you’re already well aware of this -
https://forum.huawei.com/enterprise/en/how-to-change-modbus-tcp-salve-id-from-0-to-1/thread/670233-100027
which seems to boil down to “you can’t change the id from 0 (on the onboard WiFi)”.
It’s quite possible that Huawei deliberately do this to discourage access this way.
Probably related to the onboard WiFi being in AP mode (own SSID) - how have you worked around that?
Background;
From what I can gather, the nnKTL-L1 series comes with an onboard WiFi, but it runs in AP mode (i.e. makes its own wifi network, not joining yours) for use with a proprietary app for setting up etc.
Because that’s a private network, Huawei can use any rules they like - which may be incompatible with a normal network where other systems may exist.