Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I suspect the Yammer basic network has been upgraded to Yammer Enterprise, tied to your Office365 account, that is the reason why it is being re-directed to a Office365 login.
The best way to find this out is go to your Office365 admin portal and raise a support ticket. Supply the full details of the Yammer URL, and let them do the required checking in the back end.