I think you've gotten your answer already, but I will add one tiny detail --
If your microwave is 900 watts, is that output power or input draw? Those are very different things. 900 watts output is fairly typical of medium sized microwaves, but these will typically need about 1500 watts to run.
So it is possible that you have a power problem as well as wave form problem. Note also that inverters are often rated deceptively. My Victron looks like 3000 watts, but if you read the specs carefully, it's really 3000kVA, or 2500 watts. And that's only at 20 degrees C. At normal operating temperature it's really more like 2000 watts
. Now that being said, this inverter does run everything on board without any big problems. I even run a washer/dryer off it while motoring.
It pays to read the specs carefully, before assuming what the practical output is from these units.
And in any case, as others have suggested, it would be a good idea to ditch the modified sine wave and get a true sine wave unit. Many types of equipment
will work ok on modified sine wave, but others will not, and the difference in cost is not that great, so why mess with it?