How to add a new arbitrary private IP to instance in AWS EC2 or replace a previous one?

I created an Ubuntu instance in AWS. I assigned an IP address from which I can connect to the instance from outside using PuTTY. Now I've changed my location and my IP address is different so I cannot connect to the instance with PuTTY any more. I'm trying to add another IP address (my current IP) to the instance network settings but for some reason AWS says it cannot accept the address:

IP Address not in Subnet

What does it mean? Why does it require an address in Subnet?

My questions are:

How to bypass it? or

How to change settings so that it could accept any arbitrary IP I provede? or

How to include any arbitrary IP into the Subnet of addresses? or

How can I just change previous initial address to a new one?