[lwip-users] Losing connection with router

[Mike He]

Hello,

I am having some connectivity issues running lwip, and hoping someone 
may have an idea of where to debug. I have a microcontroller running 
lwip, off the master branch, commit 
affc6d61ca7915c1dfe8a4421251875428ad5e0a. The microcontroller is wired 
to a 4G LTE router, but it is losing its connection every 12-15 hours. 
By losing connection, I mean that the router no longer recognizes the 
device. The device no longer shows up in  the list of connected hosts, 
and a long-running Websocket connection from outside the network is 
dropped. Though if I connect my computer to that network, I can still 
ping the ip address it had from when it was still recognized by the router.

Another instance of the same microcontroller has been connected for far 
longer to our separate office network, and remains connected just fine 
for as long as we've tested, which is several days. Also, a raspberry pi 
is wired to the same LTE router as the microcontroller in question, and 
remains connected for at least weeks.

I am not sure whether the 4G LTE router's behavior is nonstandard, or if 
I am doing something wrong with lwip. I tried setting DHCP lease times 
higher, and certainly they have been always configured higher than 12-15 
hours on both the router and in lwip. I have also tried sending a DHCP 
renew request every 6 hours, but the device still disconnected after 13 
hours. What protocols, settings, packets, etc keep a device connected to 
a router? What can I debug or look for to help fix the issue?

Thanks for any help, it is greatly appreciated.

Mike