Thanks to everybody's suggestions.
I fixed it this way:

I removed TEST_NODE2 as a failover member
I removed TEST_NODE1 mirror configuration
I stopped IRIS on TEST_NODE2 & TEST_NODE1
I removed all the journals as DB files IRIS.DAT are identical
I started IRIS on  TEST_NODE1 then  TEST_NODE2 
I configured the mirror on TEST_NODE1
I configured TEST_NODE2 as a failover member
Now Mirror Monitor in SMP[System Management Portal] GUI is showing no errors

Thanks to everyone for their great feedback!!

I think I used the wrong term - by "gateway" I should have used "productions"

[In System Management Portal (SMP) , GUI menu "Interoperability"-> "Configure" -> "Production"]

By "interfaces" - In IRIS command line you can run do ^SSADMIN, which has "6. Interfacing" which when selected gives me a list of "interfaces".

What is the difference between "productions" that connect systems and "interfaces" which also appear to connect systems?

I based my view on the logs I got from Node 1, Node 2 & arbiter and the timestamps.

These are some samples:

Arbiter:

2023-01-17T15:54:56.773722+11:00 LIVEARB ISCAgent[16131]: Arbiter client error: Message read failed.

Node 1:

01/17/23-15:56:22:875 (18886) 2 [Utility.Event] Arbiter connection lost

01/17/23-15:56:23:663 (24774) 0 [Generic.Event] MirrorServer: Received new failover mode (Agent Controlled) from backup...(repeated 1 times)

Node 2:

01/17/23-15:56:23:407 (9272) 0 [Generic.Event] MirrorClient: Switched from Arbiter Controlled to Agent Controlled failover on request from primary

Thanks.

I'm getting 'timed out'.

Can I get the reason why?

irissqlcli -h TRAK-BASE01 -u superuser -W -p 51773
Password for superuser:
timed out

I got the SuperServer port as follows

BASE01:~ # iris list
Configuration 'BASETC'   (default)
        directory:    /trak/base/tc/iris/basetc
        versionid:    2021.1.0.215.0.21669
        datadir:      /trak/base/tc/iris/basetc
        conf file:    iris.cpf  (SuperServer port = 51773, WebServer = 52773)
        status:       running, since Thu Feb  2 11:29:56 2023
        state:        alert
        product:      InterSystems IRISHealth

Thanks.

I have installed it on my OpenSUSE test server but when it tries to connect to the trakCare server, I get "Connection Refused".


There is no firewall running on my OpenSUSE test server nor the trakCare server.
I've tested this with firewall-cmd --stat.

The trakCare server is SUSE Linux

The main comms firewall is not blocking the transmssion either.
Do you know what ports are being used between OpenSUSE test server and the trakCare server?