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