Bringing up ethernet device under Debian riscv64

Question

I have a Sipeed RV 86 device, which is a RISCV 64bit. On this,I run a minimal Debian OS, that let's me log in via the USB interface, as the device shows up as ttyUSB0 on the host Linux PC.

When I then try to bring up networking, I stumble on this error:

# ifconfig 
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 1630  bytes 89377 (87.2 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1630  bytes 89377 (87.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

usb0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether e2:95:8c:23:89:85  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

# ifup usb0
ifup: unknown interface usb0

After making the interface known in /etc/network/interfaces.d/usb0 using:

auto usb0
iface usb0 inet dhcp

...I get:

# ifup usb0
Internet Systems Consortium DHCP Client 4.4.1
Copyright 2004-2018 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/usb0/e2:95:8c:23:89:85
Sending on   LPF/usb0/e2:95:8c:23:89:85
Sending on   Socket/fallback
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 5
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 14
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 9
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 14
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 11
No DHCPOFFERS received.
No working leases in persistent database - sleeping.
root@licheerv:/etc/network/interfaces.d# 

I know that my DHCP server is running, and working, as other clients work fine. I also see the network LED on the device blink, so there is activity.

How can I get networking going on this device?

UPDATE 1

During boot, I also see this:

[    **] A start job is running for Raise network interfaces (27s / 5min 15s)
[   33.761420] ldob: disabling

UPDATE 2

Unfortunately, the TPLINK dhcp server logging is severely lacking... it doesn't display much, other than: