I use C code to read pictures from WMTS server using DGAL.

First I initialize GDAL once:

...
OGRRegisterAll();
etc.

But new connection is opened every time I want to read new image (different urls):

gdalDataset = GDALOpen(my_url, GA_ReadOnly);

URL example:

Note that the data in native resolution for these type of services is often ridiculously large, so usually you want to specify a subset and/or limited resolution as the output.