If they're WiFi-only iPads and your AP is the only one in the area, it's quite likely that Skyhook (and other vendors) haven't geolocated your AP yet, which is preventing the iPads from performing their automatic time zone lookup. It has nothing to do with your modem/router, nor does it have anything to do with your IP address. Because of the inherent inaccuracy of IP-based geolocation, cars (like for Google Street View) drive around and record the GPS position of surrounding access points and store them in a database.
Edit: elaboration, by request:
Because not all small electronics contain GPS receivers to get their position from GPS satellites (iPads certainly don't), they rely on something called AGPS (Assisted GPS). AGPS works by using a combination of access points and cell towers (if there is a cellular chipset in the device) to derive location data. Cellular networks know very precisely where their towers are located, and through a process of triangulation, a device can pinpoint itself on a map with pretty good accuracy. In addition, WiFi is used to help location (and if you don't have a cellular device, this is the only way it can derive location). Cars drive around with very expensive, very precise measuring equipment that scans for surrounding wireless networks. Attached to this equipment is a GPS receiver, so at each sampling point, the GPS location, signal strength, BSSID (the MAC of the wireless radio on the router) and direction are recorded and kept in a database. Your iPad then looks up all surrounding WiFi hotspots and makes an educated guess on where it is located. It is less accurate than cell tower triangulation, but becomes more accurate with an increasing number of access points (for example, it's much more accurate in a downtown coffee shop than a single access point in the middle of the country).
SO, this means that your AP can't be found in the database, so it can't be correlated to a physical location, and as a result, the timezone can't be automatically configured.