LTE3301-PLUS 4G LTE-A Indoor Router - Zyxelhttps://www.zyxel.com › products_services › 4G-LTE-... is a modem/router......not a pure modem.
If the AirPort is providing DHCP addresses in the 10.x.x.x range, then you are running two routers on the same network. You only want one device providing routing services on a network. In other words, since you are trying to run two devices that are both acting as routers on the same network, this is producing what is known as a Double NAT issue along with possible conflicting DHCP range problems.
The Zyxel router is providing 192.168.1.x addresses. If you try to configure the AirPort to provide the same 192.168.1.x address range, you have two devices both trying to provide IP addresses using the same IP address range. That won't work, as you have discovered.
If you want to use the AirPort to provide routing services, then the Zyxel device will need to be modified to operate in Bridge Mode or pure modem mode, if that is possible. Then......the AirPort can be configured to use the 192.168.1.x range and things will work correctly.
Or, if you don't want to set up the Zyxel device as a pure modem.......or it cannot be set up this way........then the AirPort should be configured in Bridge Mode to turn off its routing services to avoid the Double NAT network issue.
The AirPort can still act as a wireless access point (WAP) providing a Wi-Fi signal for the network using this type of setup, but the 192.168.1.x IP address assignments will be provided by the Zyxel router. The AirPort....in Bridge Mode.....(Apple calls this "Off (Bridge Mode)".......will simply pass through the routing directions from the Zyxel router to network devices.
Other than personal preference, there is no advantage in using the 192.x range over the 10.x range, or vice versa for that matter, but its your call on that.