It seems that your problem is caused by those Iphones. And I also found a link where there is a discussion about the DHCP lease problem caused by ipad/iphone OS 3.2 in Princeton University:...

nibir.mudai wrote I have configured the modem in bridged mode and my router configured to set up PPPoE connection but this didn't solve the problem yet. The internet still drops out for about 30 secs...

ashokjp wrote See even the E1550 Huawei modem which is in the compatibility list of this router has the same issue. On lower firmwares, it disconnects in around 30 minutes Lower firmwares ? Then how...

You can send an email to TP guy for info...

Is there any anti-virus software on your win7 computer ? You can have a try to uninstall your anti-virus software and then reinstall the MFP software.

You had better contact TPLINK check your 3G modem is in their compatible list or not. If not, nobody has a solution. But you can also turn to TP tech support for advices. Good Luck!