Hello and welcome to the forum.
There are a couple species that might describe but sounds like a
Rummynose Tetra. They are definitely not fish to cycle with as they are very delicate. In any case, the 2 that died were most likely from ammonia poisoning. You really do need a test kit. For now though, start with a 50% water change and do another one in about 4 hours. After that, do a 25% change twice a day until you can get a test kit. Don't add any more fish until you have a test kit and you know that your readings for ammonia and nitrite are at 0 for 3 days in a row. When you get your test kit, make sure you get liquid and not strips. The strips are terribly inaccurate and more expensive too once you figure how many test you get for your money. Also, the 5-in-1 strips that every store sells doesn't test for ammonia which is probably the most important test in a new, cycling tank.