Remember, a DHCP lease handed to a client, doesn't go all the way to its expiry before it gets renewed. The protocol specifically says that at lease half-time, a renewal attempt will be requested.
This means for a 6-hour DHCP lease, at roughly the 3-hour mark into that lease, the client will attempt to renew that lease. If it doesn't get a response or the DHCP server is unavailable, it will wait until ~87.5%, and try again.
This page describes the process very well:
* https://lazyadmin.nl/home-network/dhcp-lease-time/
You can check locate where your DHCP leases file is, and other parameters by running the following from inside an SSH session to your RUT. Here is some sample output from mine:
# uci -N show dhcp.@dnsmasq[0]
dhcp.cfg01411c=dnsmasq
dhcp.cfg01411c.domainneeded='1'
dhcp.cfg01411c.boguspriv='1'
dhcp.cfg01411c.filterwin2k='0'
dhcp.cfg01411c.localise_queries='1'
dhcp.cfg01411c.rebind_protection='1'
dhcp.cfg01411c.rebind_localhost='1'
dhcp.cfg01411c.local='/lan/'
dhcp.cfg01411c.domain='lan'
dhcp.cfg01411c.expandhosts='1'
dhcp.cfg01411c.nonegcache='0'
dhcp.cfg01411c.authoritative='1'
dhcp.cfg01411c.readethers='1'
dhcp.cfg01411c.leasefile='/tmp/dhcp.leases'
dhcp.cfg01411c.nonwildcard='1'
dhcp.cfg01411c.dhcpscript='/usr/sbin/dhcpinfo.sh'
And the contents of my /tmp/dhcp.leases file shows:
# cat /tmp/dhcp.leases
1641822904 74:ac:b9:5a:e5:8e 192.168.1.138 * 01:74:ac:b9:5a:e5:8e
That epoch time can be converted to a regular, human-readable date with:
# date -d@"1641822904"
Mon Jan 10 08:55:04 EST 2022
That's the DHCP lease time of that interface on my RUTX11.
My two SIM cards do not have a lease expiry (here in the US, using T-Mobile and Verizon). I can also check that by running `ip a`, and looking at the `valid_lft` and `preferred_lft` time for the interface, for example:
21: qmimux0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/none
inet [REDACTED]/32 brd 255.255.255.255 scope global qmimux0
valid_lft forever preferred_lft forever
inet6 [REDACTED]/64 scope link stable-privacy
valid_lft forever preferred_lft forever