Just bought two Mikrotik-952ui-5ac2nd routers from Amazon. Neither responded to the Tiny PXE server. The two units I purchase last year, program without a problem using the same equipment and process. Tried two different computers and switches without success.
Tiny PXE returns errors unless you do that first. Once you make the factory firmware settings change you can proceed.
Ed
PS ... or of course it could be something else