Ok, spent a bit of time doing further testing just now, and still same issue occurring sorry. We are on latest version of WP 5.2 and had de-activated all other caching plugins from the network. All sites, and main site. We network activated this plugin. It activates for around 15 seconds and then de-activates itself. Unsure of cause? Also clocked issue which will stop us using it anyway. The CDN details are entered in the CDN section of WP Rocket plugin, but only apply for the 1 main site, not all sub sites. So would have to configure separately anyway. Thanks.