Yes an object the size of the ISS reflecting sunlight can easily be seen at 400 Km.
A 5 inch heliograph is visible in broad daylight at 43 miles.
Links to the transit videos mentioned in my video.
Anyone owning a P900 can use it to see if there is an object in the Sky that looks like the ISS photos NASA have shown us.
The tripod mount ring I use on the P900 is this one – it is a perfect fit.