What is the exact model number? "300tl" doesn't sound familiar.
In general, a grid-tie inverter will not operate off-grid. It has to see in-spec sine wave on input for 5 minutes, then it ramps up and shoves all available power into the grid.
Some models will work off-grid as a batteryless inverter. SMA calls that "secure power".
What can be done with some models is "AC coupling" to a battery inverter that forms a grid. It is a great solution, but costs thousands of dollars in addition to the grid-tie inverter.
SMA's compatibility list for their grid-tie inverters to be used with Sunny Island battery inverter:
I haven't found mention of "secure power", which is SMA's batteryless off-grid feature. But the similar US model does have that.
I haven't found a full manual, just data sheets.