NR5101 Bridge/Router Mode Greyed Out

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I'd like to configure my NR5101 in Bridge mode (not IP Passthrough). According to the manual there's a "Mode" drop-down in Network Setting > Broadband > Add or Edit New WAN Interface that selects between "Routing" and "Bridge". I see that drop-down, but it is set to "Routing" and greyed out. I've tried updating to latest firmware and factory resetting the modem.

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