‘Mild Hybrid’ is a misleading term- basically it’s ‘just’ a petrol engine with a slightly improved stop-start system. The car cannot move under electric power alone.
When new, or if has sat a while not being used, the supplementary battery (that’s under a front seat) doesn’t hold charge, but also doesn’t get charged until the main battery under the bonnet is fully charged again. This means it needs several, long drives to get it all back to functioning properly. This of course means that, if used mostly around town, it’s unlikely to function fully as it won’t get the long runs to charge it up.