MikroTik's, like all devices, have a chance of incorrectly generating certificates. If you are having issues with Extenders not coming online after provisioning, begin troubleshooting by logging into WinBox on the extender and checking the logs. If you see any lines that mentions certificates or failed handshakes or 'Cap failed", this fix will resolve your issues. Below are both a GUI and Command fix.

Remove Certificates from Access Points using GUI

Begin by logging into the extender/access point over WinBox. On the left-side menu, click on System Certificates. Select all Certificates and delete them. Reboot the router and log out.

Next, log into your main router and complete the same steps. Log into the main unit with WinBox. On the left-side menu, click on System Certificates. Select all Certificates and delete them. Reboot the router and log out.

Both routers will generate new certificates and will be able to communicate with each other properly, fixing the issue.

Remove Certificates from Access Points using Commands

Begin by logging into the extender/access point over WinBox or by any process so you can issue commands to the router. In WinBox, click on "New Terminal". Enter the command

 certificate remove 0,1

and press enter. Reboot the router and log out.

 system reboot

Next, log into the main unit and complete the same process. Log into the main unit over WinBox or by any process so you can issue commands to the router. In WinBox, click on "New Terminal". Enter the command

 certificate remove 0,1

and press enter. Reboot the router and log out.

 system reboot

Both routers will generate new certificates and will be able to communicate with each other properly, fixing the issue.