I think I can explain your second issue (hotspot setup). It’s probably due to routing (or lack thereof) in Device A (is it an Android smartphone?).
You say you have multiple networks behind your Asus. To explain what’s going on, I’ll assign them arbitrary network numbers. Say 10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24.
You connect Device A, via WiFi, to one of those networks, and it obtains an IP address from the Asus DHCP server. Let’s say Device A gets 10.0.1.11. Your various cameras are on any/all of the Asus subnets. Perhaps one of them is at 10.0.2.20. The Wyze app running in Device A (at 10.0.1.11) is able to communicate with the camera at 10.0.2.20 thanks to routing that is being performed in the Asus. It knows how to route packets between the 10.0.1.x subnet and the 10.0.2.x subnet (and all the other networks that you have configured on the Asus.
Now, you activate the hotspot in Device A and connect Device B to the hotspot. I don’t know the make/model of Device A, so can’t be certain of how it handles hotspots, but I’m familiar with the hotspot in iOS. An iPhone would create a new, independent LAN for devices that connect to the hotspot. I’ve done this with my iPhone, and know that it sets up a tiny wireless network with a dedicated DHCP server that leases addresses to hotspot-connected devices. That network is typically 172.20.10.0-15. (Using CIDR notation, it’s a 172.20.10.0/28). Any devices that connect to the hotspot will be assigned an address in the range 172.20.10.2-14. (Dot-1 is the address of the iPhone on the hotspot’s network, and dot-15 is the broadcast address for the subnet).
For the purposes of this example, let’s assume that your Device B were connected to such a hotspot subnet. It might get assigned the first IP in the scope of the hotspot DHCP server: 172.20.10.2.
You can now probably see where this is headed. In order for Device B to connect to the Wyzecam at 10.0.2.20, packets have to get routed correctly. Perhaps Device A has the smarts to recognize that 10.0.2.0/24 is accessible via the Asus, and knows to route packets from 172.20.10.2 off to the Asus, which will route them to the Wyzecam’s network. But even if it did, there’s no reverse path defined. Packets originating from the Wyzecam addressed to Device B at 172.20.10.2 will never get there. The Asus router has no knowledge of the 172.20.10.0 subnet. So it will route such packets off to your ISP through its WAN interface. If your ISP’s gateway router is performing correctly, it should drop these packets, since the destination address 172… is specified for use in private networks and is not routable on the public internet. (Search for RFC1918 if you are interested in such details).
There’s another possible explanation for why your hotspot setup doesn’t work as you expect, It has to do with how Wyze ‘discovers’ the network location of cameras. Too complicated to discuss here, but you might look in your Asus router to see if there is a feature named NAT looparound or loopback or redirect. If you find such an option, and it’s OFF, turn it ON and see if that helps.