Cannot connect to my instance using RDP in Windows

I am sorry I did not understand “source of alphanumeric characters”. It appears to me source type could be different security group, such as default security group, or any prefix lists. Please delete this rule and if you need the other protocol to access your EC2, just create another rule for that protocol, such as http/https or ping and save the rule, then try to ping your EC2 from your own computer. You also did not answer how your EC2 gets its public IP, such if you stop your EC2 - it may disconnect from the EIP - as far as I can remember. You have to reassociate the EIP with the EC2.

You can recheck your route table and its association also, just to confirm that the configuration did not change by accident. Finally I wish your good luck.

I have gussed that you have EIP not dynamically allowed public IP for your Windows EC2, then please check your security group rules whether the RDP protocol is allowed to connect from 0.0.0.0./0 or your own IP. If you get your Public IP dynamically (your own computer), then your Public IP can change. You can refresh your own IP or change it 0.0.0.0/0 in the security rules.

You can also check your Windows (EC2) inbound firewall and RDP protocol whehter it is closed by any chance. If it is closed open it or create a new inbound rule for RDP connection.