Please How to use code fences.
You must use sendCommand if you want the new state to go out through the binding. postUpdate only changes the state internal to OH.
Also, you should use the method, not the Action for sendCommand. This is true for 1.8 as well. https://www.openhab.org/docs/configuration/rules-dsl.html#myitem-sendcommand-new-state-versus-sendcommand-myitem-new-state
Note, 1.8 has been almost three years without an update and there are very few users left who will be able to help with 1.8 problems on this forum. Going forward, without upgrading you will largely be in your own for support. In this case the problem would also have been a problem in OH 2 so it was easy to diagnose.