A demo might help to see gameplay leaks.
The ISG itself shouldn't stop anyone from engaging it, so there's obv. something else - the indirect fire just bleeds you as intended.
Quite simply, as i said before, spread out more, and try not to force yourself too much on killing it asap.
Another way: try the Howi, the recent cost reduction makes it quite worthwhile, and they also outrange (!) the ISG. Keep in mind there's also a lot of light vehicles at your disposal. And then there's your mate... who maybe can help you to get rid of a high priority target (for you).
I felt like i did really bad that game, it was late at night and I was pretty tired. I went with my usual captain tier for the Stuart,I will definitely try out the howi since I usually ignored it before the patch and I'm still to adjusting to the new meta from the patch, he did have a sturm officer that would force retreat my infantry, which was annoying.
I know my teammate was going katushyas (which worked very well) to counter the ISGs and it was my job to protect him with my Stuart + Captain combo to force away luchs, it wasn't until I got a Sherman out until i was able to force him back hard but they eventually recovered.
I also went from playings 4v4s mostly to trying 2v2s, which are completely different of course, so I didn't expect to do exceedingly well in that game.