Mind that you need the crt/key format to use it with the Simplifier OnPremise image. It includes a preconfigured nginx Reverse Proxy. The path to place the certificates is $YOUR_PERSISTANCE/certs. The file names are also preconfigured: default.crt and default.key.
In case you want to get no browser errors and full compatibility to all mobile devices and all browsers we recommend you to buy a trusted certificate from one of the digital certificate provider.