I was doing some investigation on why our Rapido implementation wasn't showing images on some devices.
Let declaration is ECMAscript 2015 and supported by all modern devices.
However, on https://doc.dynamicweb.com/rapido/features you state that Rapido has been tested on iPhone 6s.
It seems that iPhone < 7, using the safari browser, including the 6S doen't handle let declarations.
I checked the https://www.dynamicweb.nl/campaigns/demo/demo-site website using browserstack and the image of the devices is not shown on an iPhone 6S.
My feedback to the client on this issue will be what is stated on the Rapido features page, but the statement about the iPhone6S will probably be their response on it ...
Well ... in our defence ... it says that is is tested on iPhone6S ... not what the outcome was ;)
But perhaps you should upp it to iPhone7 ?