Saving Plan for exist Instance

There are three types of savings plan and in your case, you'd either go with Compute Savings Plans or EC2 Instance Savings Plans.

Compute Savings plan applies to instance irrespective of instance region but EC2 Instance Savings plans discount would only be applied to EC2 instance usage in that particular region for which it's purchased.

So, if you buy EC2 instance savings plan for Hamburg, it'd not apply to EC2 instance launched in Frankfurt(eu-central-1) but if you purchase compute savings plan, regardless of region discount would be applied.

As you'd purchase EC2 instance savings plan for Frankfurt(eu-central-1) for t3a instance type, your instance would start getting covered under that discount.

      Compute Savings Plans provide the most flexibility and help to reduce your costs by up to 66%. 
      These plans automatically apply to EC2 instance usage regardless of instance family, size, AZ, region, OS or tenancy, and also apply to Fargate and Lambda usage.
      For example, with Compute Savings Plans, you can change from C4 to M5 instances, shift a workload from EU (Ireland) to EU (London), or move a workload from EC2 to Fargate or Lambda at any time and automatically continue to pay the Savings Plans price.

     EC2 Instance Savings Plans provide the lowest prices, offering savings up to 72% in exchange for commitment to usage of individual instance families in a region (e.g. M5 usage in N. Virginia). 
     This automatically reduces your cost on the selected instance family in that region regardless of AZ, size, OS or tenancy. EC2 Instance Savings Plans give you the flexibility to change your usage between instances within a family in that region. 
     For example, you can move from c5.xlarge running Windows to c5.2xlarge running Linux and automatically benefit from the Savings Plans prices.

Reference:

• What are Savings Plans?

• Savings Plans FAQ

I'm able to see t3a type for Frankfurt as shown below.

Hope this helps.

Comment here if you have additional questions, happy to help.

Abhishek

Hello.

The Frankfurt Region is in the list of Savings Plans.
Please select EU (Frankfurt).

You can also check the rates for the Frankfurt Region in the following document.
https://aws.amazon.com/savingsplans/compute-pricing/?nc1=h_ls