Splitting GeoTIFF into tiles in QGIS NextGIS

You could try doing as follow:

1. Get raster's spatial extension (for example going to Information tab of Raster Layer Properties dialog).

2. In Raster-->Extraction menu run "Clip raster by extent" tool selecting the raster you want to clip and choosing as extension parameters the following:

xmin=xmin_orig_raster+ m (xmax_orig_raster-xmin_orig_raster)/2 (m going from 0 to 1) xmax=xmin_orig_raster + (m+1)(xmax_orig_raster-xmin_orig_raster)/2 (m going from 0 to 1) ymin=ymin_orig_raster + n*(ymax_orig_raster-ymin_orig_raster)/3 (n going from 0 to 2) ymax=ymin_orig_raster + (n+1)*(ymax_orig_raster-ymin_orig_raster)/3 (n going from 0 to 2)

Running the tool 6 times, each with the appropriate parameters (depending on the slice you're slicing) should give you the wanted result.