The K4605 was reasonably stable on the MR3420, but was sometime picked up as a "K4605", and sometimes as a "3G Modem" - it did work, but was not 100%.
However, I then reflashed the K4605 to be a standard Huawei E372 modem, after that the MR3420 did not pick it up at all, and I couldn't use it until that Swedish guy produced a BIN file for the E372 - that is what I am using now, and it works well and is stable.
I still don't think that the MR3420 firmware is great, I wish that TP-Link would produce a better and more stable firmware version for the MR3420.
But at the moment everything works fine, although not 100% stable.
The modem does not switch off, and has no other issues - it is more of a MR3420 issue. When I use this modem in my little Asus travel router it is 100% stable, the only issue here is that the Asus has a limited wifi range (but it is matchbox sized).
Did you ever get the K4605 to work on the MR3420 properly? I might be picking one up today.
Also, is this modem stable? I don't want to get this modem if it switches off by itself or if it has any other issues.