Hi
For what its worth, my mobile router/modem a huewei is set up with the lan being 192.168.8.0/24, providing dhcp, in router mode.
My vigor 2860 , wan 2 is set to get an IP address from the router/modem automatically. the mode is "static or dynamic ip" (ie not ppoe)

Wan2 is set to take over on the failure of wan1 (broadband)
Provided the router/modem is connected , disconnecting the broadband , it fails-over ok.

The DNS settings are Google so are ok for either WAN.

Was the laptop set to DHCP, or configured to use 192.168.5.10? Probably DHCP reading it again, just the wording.

If not DHCP then that would be the issue as this would need to be configured on WAN2 on the Draytek.

If the laptop was using DHCP then I would expect the next available free IP address to be allocated (192.168.5.11) if the router was then connected to WAN2 on the Draytek.
One possible issue may be that DHCP pool may be restricted to a single IP, if this is the case a power reset of the Netgear would confirm this as the Draytek would then be allocated the IP (192.168.5.10)

The other option would be to disable DHCP on the router and set WAN manually:
IP 192.168.5.10
Mask 255.255.255.0
Gateway 192.168.5.1
DNS 8.8.8.8 & 8.8.4.4


For testing I'd also set WAN2 up for load balancing as this will be online connected and also ensure a route is in place so you can test connectivity to the 4G router through the Draytek