after further testing, it seems that the MR3420 works ok for a while, until it gets the error "Detect 3G link failed". After this error it usually dials up the modem again, and eventually connects. however, after it has connected to the internet again, pages don't load, and just keep timing out, or maybe on the rare occasions the internet pages do load, they only do so intermittently.
when I noticed this today, I then manually rebooted the router, and afterwards, the internet was running fast and stable again.
It seems that when this "Detect 3G link failed" error message appears, the router is unable to recover fully from it, and the only solution is a reboot.
I wish that this feature could be written into the firmware (automatic reboot), as this would certainly help our problem here. The problem otherwise is that, when this detect 3g link error occurs, the only option is to manually reboot the router, and many times, people here in the house, wouldn't realise this was necessary until after a while of struggling with web pages not working properly. And even if they did realise something was wrong, this would mean a lot of manual reboots, which is quite inconvenient.