Not sure if there is a better way, but I would use a loop timer to accomplish this. It would be a separate rule that you’ll trigger from your thermostat rule something like this:
To trigger this loop timer rule, you’d just set the LoopTimer variable to true to kick it off in your thermostat rule. Honestly, I would probably just set the variable to true anytime you want to set the fan to on so that way this will always get triggered. It’ll run once which should turn the fan on, and if it doesn’t turn on the loop timer will keep looping until it does.