--- How must the ZD be configured to allow remote management?
Can you try configuring ZD to assign mgmt IP/subnet in B, tag vlan B?
Configure > System > Management Interface
1. Enable IPV4 Management Interface
2. Enter IP Address/Subnet (on network B)
3. Don't check default gateway
4. Enter ACCESS VLAN (network B VLAN #)
Since switchport is trunked, client on B ought to connect to ZD ok on "new" mgmt IP.
Alternatively, try connecting from Net A to ZD on "ZD Net A IP" (what is on your "Device IP Settings > IP Address").
--- Why would the client need to be on Network A if the ZD is on a trunked switchport?
Since routing is correct, client on Net B should be able to ping ZD on Net A, right?
Perhaps your WLAN on Net B is the "Guest Access" WLAN, therefore guest access policies and control are applied (thus can't HTTPS in to ZD).
Just my 2 cents, so pardon if wrong.