can use antenna with Nighthawk AX extender for better signal

If the extender lacks external removable antennas, then attaching one can be difficult and risky since it would essentially require some hardware modification to essentially add a few RP-SMA jacks to the extender.

Beyond that, for locations with walls which heavily attenuate WiFi signals, it requires careful placement and experimentation. A good way to treat an AP in your situation is to pretend it is a super bright light bulb, and you wanted to keep the light in that room but get as much of it to reach your room as possible without getting in the way. Where in the room would you place the router?

In some condos and apartments you can end up with walls made up of thick materials that attenuate WiFi but the doors may be made with just really thin wood paneling and largely hollow on the inside. While you can't just place an AP right by a door since that will get in the way, you can experiment with placement in both rooms, or if possible, placement between 2 rooms of an extender. Also, WiFi APs will take advantage of reflected signals to improve propagation.

For example, Netgear tends to try and stay close to the 30dBm (1 watt) transmit power limit set by the FCC where possible, thus in some cases in a challenging environment where you can't run wires, you may get better results actually plating the extender outside of the room at a midpoint, and then connecting to the extender via WiFi, since the more powerful AP be able to maintain a better SNR after dealing with the attenuation of a single wall.

Another important thing to consider, make sure the verizon router, as well as the extender are not using DFS channels. DFS channels get limited to 250mw vs the normal 1000mw limit, and in a location with walls that attenuate WiFi signals heavily, that can really cause speed issues. https://en.wikipedia.org/wiki/List_of_WLAN_channels

Also make sure the G3100 is set to use 80MHz channels on the 5GHz band and not use 160MHz channel width, that will allow the use around 900mw transmit power instead of 250mw (due to the horrible DFS restrictions).

Beyond that, the Netgear extenders (as well as the ones from all other major brands) only really use their USB ports for USB storage and things like printer sharing. No WiFi router or AP will use its USB port to support a USB WiFI adapter since most USB WiFi adapters run at under 200mw transmit power, and in trying to balance physical space, and power consumption, you often end up with components that will not achieve as good of a receiver sensitivity as a purpose built AP which does not have to compromise in those areas.

I wanted to also add, if you are using a desktop PC with a WiFi adapter, you could also probably get away with not using an extender at all, and simply connecting 2 biquad or even 2 yagi antennas to the RP-SMA jacks and then pointing them at the location of the main WiFi router, and potentially also get decent speeds.