For most home cakes, yes - fan off.
The fan circulates hot air and effectively bakes HOTTER than the dial says, so the edges set and brown while the centre is still catching up. Exactly the hard-sides-raw-middle result you're getting.
The course pattern for OTGs: where a professional deck oven bakes at 200°C, an OTG does the same job at 180-190°C - and with the fan on, you drop further still. If tops or sides keep overdoing even then, switch to lower-rod-only at reduced temperature.
The fan isn't useless - it's good for even browning on trays of cookies and for roasting. It's just the wrong tool for delicate batters that need time to rise before the walls set. Rods for cakes, fan for cookies, and your thermometer as referee.
