A firewall in transparent mode has no Layer-3 route. How to remotely manage the firewall using Telnet? If the IP address route is reachable, the device can be remotely managed using Telnet.
A system IP address cannot be configured on the USG5300 in transparent mode. To remotely manage the device using Telnet, do as follows:
L3------USG5300 (transparent mode)------L3. Configure a sub address at Layer 3. For example, configure a sub IP address, 126.96.36.199/30 sub, for the USG5300 interface at Layer 3, and configure a VLAN, for example, VLAN 100, on the USG5300. VLAN 100 contains two interfaces on the USG5300 connecting to Layer 3. Set the Layer-3 Vlanif 100 IP address to 188.8.131.52/30 at interface vlan 100. This IP address is reachable through routes on the entire network. Then the USG5300 can be managed remotely.