How do I access my modem's GUI when it's in bridged mode?
The few i've used, when in bridge mode, have no web interface or IP. (maybe yours does)
With those, you have to reset it(holding the reset pin) to get to the web interface and that takes it out of bridge mode. With yours, there may be other ways, but You can reset it too like that and get out of bridge mode and to a web interface.
no way to access the modem GUI if I want to do things like turn off
its wireless antenna. (I was also thinking about disabling its NAT and
DHCP functions, although are those are even relevant when it's in
With ones i've used, NAT and DHCP definitely aren't done when it's in bridge mode, I can't comment on others. I'd be surprised if it did wireless but you can check just to make 100% sure.. If you know the SSID (which you'd need to know to know how to connect to your own router when it's in regular mode), then look for that SSID to see if it's broadcasting. I'm sure you'll find that's off too, in any of these devices when in bridge mode. I suppose you could also prove that it's not doing NAT, with the tracert command tracert www.google.com and look for private IP addresses like 192.168 or 10.0 and you should only see one of them, if double NAT I suppose you'd see two.
I have edited my answer, my previous answer said "When it's in bridge mode it has no web interface, no IP." I removed that since it's false for DDWRT and possibly false for some makes/models. The other answer says no , however that's not true either. I know this to be the case(the web interface turning off when in bridging mode), with some router/modem/switches, specifically with at least some, belkin ones e.g. http://forums.whirlpool.net.au/archive/869904 as mentioned here "if the belkin is correctly set to bridge mode you can not enter its gui web interface , as it is just a modem". It is true of DDWRT that you can still access the web interface.. Though quite likely that his particular make/model can, as the other answer claims to have used similar models to the OP, and says you can, i.e. you can with his. Though the other answer then talks about DDWRT which the OP may not be running. And the other answer doesn't specify what is going on with the OP's device if he has no access to his web interface.