Tried in multiple ways and failed to set up certificates on Shelly Plus with v0.14.1 so that AWS IoT would work. If I remember correctly you also had to set the parameter use_client_cert to true from Mqtt.SetConfig, but even with that we could not get it to work. Also couldn't find any solutions with some Googleing.
Maybe it's fixed in v1.0.3 firmware, haven't tried.