EC2 Instance Connectivity Issue Despite Instance Running Successfully

Hello.

Can you verify if there are any network or security group issues that might be preventing inbound SSH connections to my instance?

Can you confirm that there is a route to the internet gateway not only in the security group but also in the route table of the subnet where EC2 is running?
Even if a public IPv4 address is attached to EC2, it cannot be accessed unless a route to the internet gateway is configured.
Also, if you are experiencing a timeout error, the troubleshooting steps in the document below may be helpful.
https://repost.aws/knowledge-center/ec2-linux-resolve-ssh-connection-errors

Is there any specific AWS service health issue affecting connectivity in the [insert AWS mumbai region that could be causing this problem? Are there any logs or diagnostics that could provide insights into why SSH connections are not reaching my instance despite it being in the running state?

If the EC2 status is OK, nothing will be displayed on the AWS health dashboard etc.
Although it cannot be used with t2.micro, there is a way to check the system log.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/troubleshoot-unreachable-instance.html

How can I convert my current AWS Free Tier usage to paid usage, considering my instance is now in the pay-as-you-go pricing model? How do I utilize the startup grant provided by AWS in my account to cover the costs associated with my instance usage?

In other words, if you run EC2 for more than 750 hours on an instance type eligible for the free tier, such as t2.micro, you will automatically be charged.
https://aws.amazon.com/free/?nc1=h_ls&all-free-tier.sort-by=item.additionalFields.SortRank&all-free-tier.sort-order=asc&awsf.Free%20Tier%20Types=*all&awsf.Free%20Tier%20Categories=*all

Startup support requires participation in AWS Activate.
https://aws.amazon.com/activate/activate-landing/?nc1=h_ls

You will receive credits by participating in AWS Activate, so you will need to apply them to your AWS account by following the steps in the document below.
https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/useconsolidatedbilling-credits.html

Can you verify if there are any network or security group issues that might be preventing inbound SSH connections to my instance?

Nobody within the AWS re:Post community has access to your account, so this question can't be answered.

You provided the public IP address of your instance, and I get a Connection refused when I try to SSH to it. This proves that a process is listening on port 22 of the EC2 instance, and that is almost certain to be SSH, but it is not accepting connections.

$ ssh 43.205.64.245
ssh: connect to host 43.205.64.245 port 22: Connection refused
$ ncat -vz 43.205.64.245 22
Ncat: Version 7.93 ( https://nmap.org/ncat )
Ncat: Connection refused.
$

(Obviously I don't know your password and I don't have your provate key so there's no chance of forcing my way onto your host).

This suggests that the networking infrastructure - subnet, security group, route table, gateway - is correct, and that the problem is likely to be witin the Linux side of the running EC2 instance itself.

I'll split this out into a separate answer, as it's more related to billing than a technical query.

How can I convert my current AWS Free Tier usage to paid usage, considering my instance is now in the pay-as-you-go pricing model?

If your account is in its first 12 months, and the instance type is t2.micro (or t3.micro in some regions) then it will be in-scope of Free Tier. Any other instance type is out-of-scope of Free Tier, as are all instance types on accounts older than 12 months.

As you might expect, Free Tier instances tend to be low-powered, and are intended to allow new users to try out new things for low (or no) cost. If you find these low-spec instances don't meet your needs then you are free to uplift to a better instance https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html

When AWS counts up your usage at the end of each month and generates your bill and sends you an invoice, it will apply Free Tier allowances where appropriate. There is no difference between a "free" t2.micro and a "paid for" t2.micro, they both have exactly the same specifications.

There is a difference between a t2.micro and (say) a t2.xlarge https://aws.amazon.com/ec2/instance-types/