iPhone 11 Charging Slow

So I recently got a new iPhone 11 which I believe to be capable of charging speeds up to 18W, although that requires the USB-C fast charger from apple. I do, however, have an old 10W iPad USB type A charger and a third party 12W one. However, after some testing these chargers don't seem to charge the phone any faster than the included 5W. All usually charger the phone 4-5% in 5 minutes or so no matter the wattage. I've tried different lightning cables, plugging directly into the wall instead of using the extension lead by my bed, and turning off optimised battery charging but no matter the higher wattage chargers don't decrease charging time they're all always the same or within a few percent. I've tested with short periods and longer periods of time and counting battery percentage and it never goes up any faster despite using the more powerful chargers.

I've come here as a last resort after trying everything, thanks.