Reestablish client to server relationship

Hi team,

As a result of one of my networks not being able to contact the SEPM server for months all the clients are now showing as offline in the management console. I've adjusted firewall rules and now the requried ports are open I need them to start checking in and picking up updates again, however they all remain in an offline state.

I've tried repairing the install and pushing out the client to these servers, however they still don't check in. I've even tried a full uninstall and reinstalled, the client now shows good (green circle icon on it) however on the management server it's still showing as offline!

What is the process for getting these clients checking in again?

Thanks