WAN DHCP issue after link down+ link up

[km400a]

I changed my ISP to cable modem in this week.
my WAN use DHCP to get a IP form WAN.
I find the WAN is unstable, it may link down once a day.
after link down then link up in WAN, I can't access internet except my router issue DHCP renew request. I use this way manually to fixed it.
Can udhcpc reissue DHCP renew request after Link down+ Link up?
does it possible to update /lib/netifd/dhcp.script to support this feature?

I also do some test by my slef as below.
Test 1: remove the WAN cable then plug-in
In this test, remove WAN cable causes link down, after I plug-in the WAN cable then Link up.
the internet access is ok after link up. no need DHCP renew request.
I think this is correct behavior of my router and cable modem.

Test 2: power off then power on the cable modem
in this test, the WAN is also link down then link up in my router.

use "ip addr" show my WAN is Link up
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc hfsc state UP qlen 1000
link/ether 90:f6:52:b7:47:2d brd ff:ff:ff:ff:ff:ff
inet 213.121.241.48/24 brd 213.121.241.255 scope global eth1

but I can't access internet.
I think this issue is the cable modem had lost DHCP information after power off.
Then I can manually send dhcp renew request to make the WAN can access internet again.

below is the log from "logread" when issue DHCP renew request
Aug 24 07:43:42 Gargoyle daemon.notice netifd: wan (7081): Performing a DHCP renew
Aug 24 07:43:46 Gargoyle daemon.notice netifd: wan (7081): Sending select for 213.121.241.48...
Aug 24 07:43:47 Gargoyle daemon.notice netifd: wan (7081): Lease of 213.121.241.48 obtained, lease time 21600

I suspect my original issue is like Test 2, my cable modem maybe have a bug then lose DHCP information, that's why I need to reissue DHCP renew request to make it work.
if the link between my router and cable modem have compatible issue then cause link down+ link up just like Test 1. it should no need DHCP renew request.

so, If I want to patch this cable modem issue(I suspect). Please help me how to do. Or you think I need to ask my ISP to fixed it?

[DoesItMatter]

What router?

What cable modem? Cable modem company?

Can you get a stable connect from 1 PC direct to the cable modem?

[km400a]

What router?

What cable modem? Cable modem company?

Can you get a stable connect from 1 PC direct to the cable modem?

my system information is below
Gargoyle Version:1.5.10
Model:TP-Link TL-WR740N/ND v4

Cable modem is CISCO, Model DPC2100R3
Cable modem company is http://www.araedis.tv/ a Taiwan local company.

I had checked the Test 2 again by using BUFFALO WCR-G300 -> the AP is provided by my ISP. The result is ok. below is the log from BUFFALO WCR-G300.

It always renew DHCP after Link down + Link UP. and it works fine without link down in 24 hours. I will keep tracking it.
note: port 5 is WAN of AP.

Aug 24 20:22:53 DHCPC Lease of 213.121.241.140 obtained, lease time 21600
Aug 24 20:22:53 DHCPC Received DHCP update...
Aug 24 20:22:52 DHCPC Sending SELECT for 213.121.241.140...count(1)
Aug 24 20:22:52 DHCPC Enter REQUESTING state...
Aug 24 20:22:52 DHCPC Received DHCP OFFER...
Aug 24 20:22:51 DHCPC Sending DISCOVER ...count(1)
Aug 24 20:22:50 DHCPC Entering INIT state...
Aug 24 20:22:50 DHCPC udhcpc (v1.12.1) started
Aug 24 20:22:49 WIRED Port 5 Link UP !!
Aug 24 20:22:37 DHCPC udhcpc EXIT...
Aug 24 20:22:37 DHCPC udhcpc EXIT...
Aug 24 20:22:37 DHCPC Received SIGTERM...
Aug 24 20:22:37 DHCPC No lease, forking to background.
Aug 24 20:22:37 DHCPC DHCPC timeout...
Aug 24 20:22:36 WIRED Port 5 Link DOWN !


I think I need to find a way make my router in Gargoyle do the same way.