could you tell me which IDE you use to Build?

We are developing a USB device which, among other interfaces, provides ethernet via NCM with the help of your great library. It works beautifully.
However, we are running into a problem where the host sometimes decides to disconnect our device. We tested with other ethernet devices as well and saw the same disconnection behavior, so our device is not at fault (or at least shares this problem with others).
We are looking for a way (possibly a callback) to detect when the host disconnects from us. Unfortunately we cannot simply check VBUS as this pin is not handled by our main processor directly. Is there any way to ask how long ago the last transaction took place? We learned that not all transactions raise interrupts, so simply resetting a timer in the isr (as I ignorantly tried) won't do.
Any help is greatly appreciated.

On Linux ethernet usb0 device shows up, but DHCP fails to get an answer.
Similarly on Mac - device shows up, but DHCP fails to get an IP address.

Below are included relevant logs from a Fedora 32 Linux box (the pcap has .log extension since github doesn't want to allow .pcap).

This is a FreeRTOS build of IPoverUSB.

diff --git a/Makefile b/Makefile
index 4a27072..aecde89 100644
--- a/Makefile
+++ b/Makefile
@@ -16,7 +16,7 @@ XPD_DIR = STM32_XPD
USBD_DIR = USBDevice
LWIPDIR = lwIP/src
CONTRIBDIR = lwip-contrib
-OS_DIR =
+OS_DIR = FreeRTOS/FreeRTOS/Source

##++---- Included files ----++##
include $(LWIPDIR)/Filelists.mk

This is using a:
https://www.amazon.com/gp/product/B00CW9AKDY/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
[STM32F4DISCOVERY] ST STM32 STM32F4 STM32F407 MCU Discovery Evaluation Development Board kit embedded ST-LINK/V2 debugger @xyg

Best guess is Linux/Mac and the IPoverUSB device are not agreeing on USB URB (possibly MBIM) framing and thus frames don't flow successfully in either direction.

[ 0.000000] Linux version 5.8.9-200.fc32.x86_64 ([email protected]) (gcc (GCC) 10.2.1 20200723 (Red Hat 10.2.1-1), GNU ld version 2.34-4.fc32) #1 SMP Mon Sep 14 18:28:45 UTC 2020
...
[316478.147751] cdc_ncm 1-6:1.1 enp0s20f0u6i1: unregister 'cdc_ncm' usb-0000:00:14.0-6, CDC NCM
[316484.539176] usb 1-6: new full-speed USB device number 7 using xhci_hcd
[316484.666861] usb 1-6: New USB device found, idVendor=0483, idProduct=ffff, bcdDevice= 1.00
[316484.666873] usb 1-6: New USB device strings: Mfr=16, Product=32, SerialNumber=48
[316484.666881] usb 1-6: Product: IP over USB Demonstrator
[316484.666886] usb 1-6: Manufacturer: STMicroelectronics
[316484.666891] usb 1-6: SerialNumber: 19004100
[316484.690225] cdc_ncm 1-6:1.1: MAC-Address: 00:80:e1:00:00:00
[316484.690671] cdc_ncm 1-6:1.1 usb0: register 'cdc_ncm' at usb-0000:00:14.0-6, CDC NCM, 00:80:e1:00:00:00
[316484.708297] cdc_ncm 1-6:1.1 enp0s20f0u6i1: renamed from usb0
[316552.462093] device enp0s20f0u6i1 entered promiscuous mode
[316552.463409] cdc_ncm 1-6:1.1 enp0s20f0u6i1: 10 mbit/s downlink 10 mbit/s uplink
[316552.479401] cdc_ncm 1-6:1.1 enp0s20f0u6i1: network connection: connected
[316552.479457] IPv6: ADDRCONF(NETDEV_CHANGE): enp0s20f0u6i1: link becomes ready
[316619.602751] device enp0s20f0u6i1 left promiscuous mode

ip link set enp0s20f0u6i1 up; tcpdump -s 10000 -w dump.pcap -i enp0s20f0u6i1

tcpdump -r dump.pcap

reading from file dump.pcap, link-type EN10MB (Ethernet)
dropped privs to tcpdump
17:07:47.493508 ARP, Request who-has 192.168.0.1 tell 192.168.0.1, length 28
17:07:47.495812 IP6 :: > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28
17:07:47.606866 IP6 :: > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28
17:07:48.438819 IP6 :: > ff02::1:ff00:0: ICMP6, neighbor solicitation, who has zeus.lan, length 32
17:07:49.462910 IP6 zeus.lan > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28
17:07:49.462926 IP6 zeus.lan > ff02::2: ICMP6, router solicitation, length 16
17:07:49.958865 IP6 zeus.lan > ff02::16: HBH ICMP6, multicast listener report v2, 1 group record(s), length 28
17:07:53.110865 IP6 zeus.lan > ff02::2: ICMP6, router solicitation, length 16
17:08:00.726869 IP6 zeus.lan > ff02::2: ICMP6, router solicitation, length 16
17:08:15.574873 IP6 zeus.lan > ff02::2: ICMP6, router solicitation, length 16
17:08:21.490129 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:80:e1:00:00:00 (oui Unknown), length 300
17:08:27.113025 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:80:e1:00:00:00 (oui Unknown), length 300
17:08:38.358633 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 00:80:e1:00:00:00 (oui Unknown), length 300
17:08:46.294864 IP6 zeus.lan > ff02::2: ICMP6, router solicitation, length 16

dhclient -4 -d -v enp0s20f0u6i1

Internet Systems Consortium DHCP Client 4.4.2b1
Copyright 2004-2019 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/enp0s20f0u6i1/00:80:e1:00:00:00
Sending on LPF/enp0s20f0u6i1/00:80:e1:00:00:00
Sending on Socket/fallback
DHCPDISCOVER on enp0s20f0u6i1 to 255.255.255.255 port 67 interval 6 (xid=0xbb3dfe7c)
DHCPDISCOVER on enp0s20f0u6i1 to 255.255.255.255 port 67 interval 11 (xid=0xbb3dfe7c)
DHCPDISCOVER on enp0s20f0u6i1 to 255.255.255.255 port 67 interval 18 (xid=0xbb3dfe7c)
^C
dmesg.txt
dump.pcap.log