Dhcp server change gateway and dns

I have a cpe510 configured as an ap client with ip .
I would like to activate the dhcp server on this device but I would like the gateway assigned to the clients to be and the dns was .
How can I do?

I suppose your goal is to have the lan of the cpe510 and the wifi where it connects to in the same broadcast domain. Is that correct?
If so, how are you going to implement the wwan on cpe510? WDS, relayd, or some other way?

The lan and the wifi are already in the same network, the devices connected to the cpe510 receive ip from the adsl router .
Sometimes the devices connected to the cpe510 do not receive the ip, I would like to try to activate the dhcp server on the cpe510.

If the cpe510 and the other router are in the same broadcast domain, you shouldn't enable 2 dhcp servers simultaneously, that might cause conflicts.
Better post here the following to understand the situation better.

uci export network; uci export wireless; \
uci export dhcp; uci export firewall; \
head -n -0 /etc/firewall.user; \
ip -4 addr ; ip -4 ro ls tab all ; ip -4 ru; \
ls -l  /etc/resolv.* /tmp/resolv.*; head -n -0 /etc/resolv.* /tmp/resolv.*

Please use "Preformatted text </>" for logs, scripts, configs and general console output.

I don't have the CPE510 here, I'm aware of the risks.
I will configure the dhcp of the adsl router from 100 to 150 and the dhcp of the cpe510 from 151 to 200.

I guess just add 2 options here:

config dhcp 'lan'
        option interface 'lan'
        option dhcpv6 'server'
        option ra 'server'
        option ra_management '1'
        option leasetime '48h'
        option force '1'
        option start '141'
        option limit '57'

That is a bad idea and judging by the options that you use in the snippet above you don't fully understand what you are doing.
It makes more sense to troubleshoot why the dhcp fails with the main router, rather than add another dhcp server.