Hi @dazmatic 

Thanks for posting in our business forum.

Simple as this.

If this does not answer your question, glad to discuss this after you take a look at this article.

And hope to see some replies with Wireshark results and screenshots of your WAN info if you need to discuss.

Please mosaic your sensitive information. Here is a list of information considered sensitive:

1. Public IP address on your WAN if your WAN is.

2. Real MAC address of your device.

3. Your personal information including address, domain name, and credentials.

For troubleshooting purposes, when a WAN IP is needed, please leave some values visible for identification.

  @dazmatic 

This is actually a limitation of the modem firmware and not the TPlink router. 

IP passthrough does some magic with DHCP, and there are in fact two loops, Carrier<->Modem and Modem<->Router. 

What you are asking for is Carrier<->Router, which is how 'bridge' mode typically works but cellular modems (as opposed to wireline modems) typically use 'IP Passthrough'.  The problem comes down to how often the modem updates the LAN device with DHCP (DHCP lease times, etc etc.). When you force a reconnect, the modem should detect the change in the IP being assigned to its WAN side, and then duplicate that change via local DHCP to the LAN device.  If you are able to configure the DHCP parameters on the 4G modem, try setting DHCP lease times to something like 2 min.

If anybody writes a script it will need to be you, running on a 24/7 machine behind the router.  Script will need to detect WAN down, and then bounce either the modem power, or the modem link.

  @dazmatic 

I hear your frustration, but you are fighting the 'link down' detection which works based on the router pinging an IP (router->Config->Advanced->Echo Server) versus the WAN interface IP which typically works on etherlink down, or IP lease expiry.  In your case, your router thinks it has a happy WAN IP (but doesn't because the carrier changed it while the previous IP lease time is still good--technically this is a mismatch between the carrier and modem settings). 

So either the modem, does a refresh cycle (supports RFC3203, DHCPFORCERENEW), shrinks it's lease time to 2min, or actual electrically bounces the etherport on IP change.  Anyways, ball squarely in Zyxel's court.  Since that's highly unlikely to happen, you're probably better off building it yourself via docker/vm/pi.  Alternatively, you may be able to find a 4G device that does a better job of IP Passthrough.

Lastly, you could submit via the Requests forum a DHCP WAN lease time override (ie modem says 6h, tplink router overrides that to 2min).  This wouldn't be a great deal of effort to add to code.