You'll need a radius server with an attached database to take care of the accounting. You can either use the MikroTik User Manager (separate package to ROS) which runs on top of your routerboard or simply use an external radius server running FreeRadius + MySQL/MariaDB/PostgreSQL. A word of advice - the MT user manager is quite a pain and requires a lot of patience to set up while IMHO an external solution usually is considered less stressful. As an alternative to a self-configured freeradius instance you might want to take a look at RadiusManager from DMA SoftLab:
It isn't pretty but gets the job done.