My ISP says the modem/router combo that they supply is in bridged mode, but it's wifi address still functions. Is this possible? I am having all kinds of issues with wifi dropping on my Nest network, and this seems like the only possible cause. There is no modem only product for 1gig speeds (with phone jack) offered for Optimum.
Technically, yes – it could be in bridged mode and still be a WiFi access point, but that is extremely unlikely to actually be supported by any modem. Another clue is that the device has a phone jack, which I assume means it is providing your home phone service. That typically requires the device to be in router (not bridge) mode as well. It may be possible to verify it by going to a site like https://www.whatismyip.com/ and noting what IPv4 address the rest of the world thinks you have, and then comparing that to the WAN IP address your Nest WiFi Router has (Google Home app, tap on your Nest WiFi Router, then tap the gear icon in the upper right corner – WAN IP is under "Wi-Fi information"). If they are the same (meaning the IPv4 address shown on the what is my IP web site and the WAN IP address in the Google Home app), then your modem is in bridged mode. But, if they are different, then your modem is not in bridged mode and you have double NAT.
Aside from the router/bridge question, having another WiFi access point placed very close to your Nest WiFi Router can cause problems, too. So, you may want to try just moving them apart – six feet / two meters should be enough to improve things if that is the issue.