Original Factory ROM file :
18-02-02 t900a-dg-a59-63m-hd-512g48g-bom6-3m-n-tee-cts_DOOGEE-MIX-Helio_P25-20170818.rar (6/64Gb)
There is no special loader for doogee, all you need to do is
Hold volume up button and connect your phone to PC via USB cable.
Pay attention to preloader compatibility on doogee phones,
or, to be sure not to make it a brick - do not flash preloader.
p.s. Enable checksum check on flashing.
b.r.
Alex