According to the model number you have a hot surface ignitor ignition system. You need to watch the start up sequence. It should go something like - Thermostat calls for heat, draft inducer motor runs, draft inducer achieves needed speed to satisfy pressure switch, Hot Surface Ignitor turns on glowing white hot, gas valve opens, burners ignite, 45 seconds later main blower starts.
I think you may have a bad board or at least a stuck-closed main blower relay. But see how far in the ignition sequence your furnace gets. Maybe there's something we've missed. But also locate a place to get the board. Also remove the board and look for fried components or cold solder joints to confirm your suspicions