Unable to add arbiter node from another region(AWS)

Hi,
I’ve a working replica set on aws using ec2 instances where 2 instance in us-east-1(one primary) and 2 on us-west-2. I’m trying setup 3 region replica set to address split brain situation.
Though I’m creating 3rd region(ap-south-1) replica with latest AMI backup from us-east-1 region, still as soon as I’m adding the the arbiter instance it is showing me invalid conf.

	{
		"_id" : 5,
		"name" : "192.168.1.21:27017",
		"health" : 0,
		"state" : 8,
		"stateStr" : "(not reachable/healthy)",
		"uptime" : 0,
		"lastHeartbeat" : ISODate("2025-04-21T06:29:48.556Z"),
		"lastHeartbeatRecv" : ISODate("1970-01-01T00:00:00Z"),
		"pingMs" : NumberLong(0),
		"lastHeartbeatMessage" : "Our replica set configuration is invalid or does not include us",
		"syncingTo" : "",
		"syncSourceHost" : "",
		"syncSourceId" : -1,
		"infoMessage" : "",
		"configVersion" : -1
	}

any suggestions, what can be the missing part here?