None of SIP clients can log in to a new instance

In this new instance I only want SIP clients on the LAN to be able to place voice and video calls. For now, this is not going to be accessible from the outside, i.e. not calling out to the real-world phone network, not receiving any calls.
So, my LAN is 10.0.0.0, and FreePBX 17 is installed on Debian 12 Linux at 10.10.10.10. Its domain name is freepbx.local.domain.
In this instance, I’ve created ext. 200 and a new user for it called login0 with password login0.
I tried both with the firewall allowing SIP TCP and UDP allowed on Local/Internet/Other and with the firewall stopped.
Tried from a Windows machine on which the firewall allows 5060 and 10000-20000 ports to 10.10.10.10 and from a Linux machine that has no firewall. Equally, MicroSIP from Windows and Linphone and Zoiper from the Linux machine cannot connect. MicroSIP behaves weirdly: at times it instantanously says ‘Wrong password’, at times it times out. Zoiper shows this:
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Tried entering login as [email protected], [email protected], and login0. No difference.
Tried to enter server as freepbx.local.domain and 10.10.10.10. No difference.
Am I missing some steps on FreePBX that have to be performed, to allow softphones to connect? What are they?

What is your netmask ?

Mine? Not sure. Never worn one.
The netmask on my LAN is 10.0.0.0.

For both 10.0.0.0 and 10.10.10.10 to be in the smallest subnet you will need to use 10.0.0.0/12 or 255.240.0.0

I do not follow. Why is this relevant? Why should I worry for something to be in a smallest subnet?
Are we still on the topic of softphones not registered, or have we switched gears for something completely different?

You are correct, I originally misinterpreted your post.

The user management user is unrelated to SIP registration. Browse to Connectivity, Extensions and select the extension you want to register to. There you’ll see the SIP secret and port, which together with the extension number and the domain/ip will form your SIP registration details.

Still I get 401:
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What else do I need to put in softphones for them to register?

How specifically will they?
This is its login/secret:
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