Debating whether to put my ISP router in bridge mode or my airport time capsule.

I want to use my airport time capsule as the router and essentially use the ISP gateway as ONLY a modem (by disabling it's WiFi and putting it in bridge mode)

In theory, It does not really matter whether the ISP gateway is configured as a router and the Time Capsule is set up in Bridge Mode......or......whether the ISP gateway is set up as a modem only and the Time Capsule is configured as the network router.......unless.......there is some special feature or setting that you want to use that would require that the Time Capsule be configured as the router for the network.

Frankly, I would be surprised if your ISP gateway can even be configured to act as a simple modem, even if you wanted to set it up that way, so you might want to check with the support folks at your ISP to ask about this. Even if it can be set up as a simple modem, your ISP is probably going to be a lot less than thrilled about helping you to do this.

Another possible issue is that your ISP: may not support this type of setup, so if there are issues with the Internet connection, your ISP will look the other way.

Final thought......even if the gateway can be set up in Bridge Mode, the Time Capsule will need to connect to your ISP signal using what is called PPPoE credentials. But......much of the UK uses what is known as PPPoA.....which is not compatible with the Time Capsule.

Life would be lot easier if you left the ISP gateway "as is" and simply change the Router Mode of the Time Capsule to Bridge Mode, which Apple calls "Off (Bridge Mode)".

Double NAT is not the end of the world and many users run things this way, so if things are working well now, you might also consider leaving well enough alone. Apple would recommend the Double NAT setup for what that is worth.

However, if you are having problems because of the Double NAT, then that would be a good reason to switch the Time Capsule to Bridge Mode.

One final question, since I am also using multiple airport expresses to wirelessly extend my network, do these also need to be in bridge mode, because currently they aren't I don't think. If so, I would assume they are triple NAT-ing, which can't be good.

If the AirPort Expresses are set up to "extend" the WiFi network, then you can be assured that they are already set up in Bridge Mode, since they could not extend the network otherwise.

Use the DHCP setting for the connection if things are working. Static could be used if you find that the Time Capsule is losing its Internet connection more often than you would like.

If you go to Static for the Time Capsule, then you should also go to Static for the Expresses.

Use Link Local Only for the IPv6 setting since the ISP gateway is already handling IPv6. You cannot turn IPv6 off on the AirPorts.