Regarding the Axpert. Been in long debates about this. Made a lot of people very upset. Even got banned from commenting on Sokolic Solar’s Facebook page for (very respectfully) asking about it… 
Apparently the way to do it is just to declare yourself as “off-grid”. Is it legal? Don’t know, probably not. If anyone actually shows up and polices it, you will probably have problems.
Does the inverter itself comply fully, if we do our own “inspection” and ignore the lack of paperwork? Well, some of them didn’t even have proper TN bonding at one point, so the answer for a large number of them was a definite no.
But if they have TN bonding? Well, the regulations say the changeover must be interlocking. It must be impossible to have both the inverter and the grid connected simultaneously. The last time I checked this was also not the case for the Axpert. It doesn’t even disconnect the neutral of the inverter while connected to the grid, only live.
But it is still acceptable as a UPS for some reason. The really tough rules only apply when you add PV modules. Which is weird, but that is how it is