WMS tiles display in the WEB version, but not ANDROID

I have a set of WMS maps that I use in my own data space. These display correctly and scale correctly in the web-based version of CalTopo / SARTopo (even when run in Chrome on Android), but flat-out refuse to load on the Android app. This is extremely frustrating at these are the national 1:50,000 topos for South Africa. 

Any suggestions? 

The server that provides them is at:
http://apollo.cdngiportal.co.za/erdas-iws/ogc/wms/CDNGI_PORTAL_BACKDROP?service=WMS&request=getcapabilities 

Many thanks for