I just recently acquired a pre-loved late-2015 21.5 iMac with A1644 keyboard. All worked 100% till just now when the 1 and 2 numeric keys stopped working. Not on their own nor when combined with shift. No liquid spills or the like. The overall condition of all components is like new - extremely well looked after. I’ve given it a shake, light tap in an attempt to dislodge whatever could possibly be stuck underneath it, but to no avail. Please help…