irfangul

I wonder if they really did.
It sounds like the battery-side capacitors have dried out in the Pakistani heat (guessing you're in Pakistan, that seems to be the only country where Inverex branded inverters are sold). These capacitors (there are 4 of them in a 5 kVA model) prevent spikes on the battery-side from blowing the MOSFETs. It looks like some of the driver transistors were also affected. It can be quite a job finding all the blown parts and replacing them all; if you miss just one, it can blow again. But that's usually immediately. With the one week delay, that sounds like the capacitors are weak, and are absorbing many of the spikes, but one in a billion (?) cycles they're not good enough, and the MOSFETs blow.
Personally, on all my inverters, before I use them I replace these capacitors, which are rated at 2000 hr @ 105°C, with slightly more expensive types rated at 10 000 hr @ 105°C, and the MOSFETs with higher voltage types. It all comes to about AU$100 (US$70). Doing this early means you don't have to find all the blown gate driver components. I've had no problems, but the climate here in Brisbane, Australia is probably milder than the climate in Pakistan. Though we happen to have 39°C forecast for tomorrow, and the start of summer is still weeks away.