I fried my spindle motor, and would like to understand why

[JAZZCNC]

The motor states 300hz max.
This kind of thing happens when you mix and match.
Should have got a matching vfd from isel.

May have broke both.

No, it doesn't, Isel doesn't make VFD's they just supply another manufacturer VFD that is set up to match their spindle. Any suitably sized VFD with the correct setting will work fine, with any spindle from any manufacturer.

The problem could be a number of things and like John says most probably faulty spindle in the first place or incorrectly wired because just entering the wrong frequency wouldn't burn it out within 3s, it would squeal and complain but wouldn't kill it dead.

The base Frequency should have probably been 50hz or 60hz to suit your country's grid power. The Motor rating of 300Hz is the Rated HZ which will provide the rated power and going higher would just make it spin faster but with lower power, but go too high and you would get overheating, etc.

My suggestion is to contact Isel and ask which VFD they use and what settings they use.