cannot reach our server

Sorry to hear you are having issue with the instance. This is better be handled by submitting a case where a support engineer will have proper tool to dive deeper to assist you. Also I would recommend to not expose any resource information such as Instance ID for security and privacy concern. If you haven't submitted a case in the past, below is the "how-to" instruction.

https://docs.aws.amazon.com/awssupport/latest/user/case-management.html

If you changed the Route 53 record from the IP of the instance in question to another one and then back, then tried to connect to the first instance using the domain name and it didn’t work, it could be due to the time it takes to update your DNS information.

Route 53 uses TTL to define how long (in seconds) DNS recursive resolvers should cache information about the record. After changing a Route 53 record, you may need to wait before connecting to let the TTL expire, so that the DNS recursive resolvers will have to query Route 53 again to update their cache with the new IP to which you pointed the record.