Where is the best place to enable DHCP?

Question

Network administration

There is an adsl modem (without WiFi), a router with WiFi is attached to it, which distributes the Internet to all computers and devices. When configured on both devices, DHCP was enabled by default, but the Internet behaved very strangely: the computer through the wire and the computer through WiFi worked fine, and the tablet caught the signal through WiFi after a couple of minutes of watching the video, but access to the Internet disappeared until the network was manually deleted and new connection.
I went into the router settings and unchecked DHCP, after which everything seems to be fine for now. Who understands this, tell me what could be the problem and whether it depended on this option? Did I do the right thing by disabling DHCP on one of the devices? Was it right to turn it off on the router, and not on the modem?
Thanks in advance
UPDATE
In the end, what happened? It seems that everything is set up, approved by competent people, but still something did not take off. On half of the devices, the Internet works fine, others constantly lose connection and require Wi-Fi to be restarted. I messed around a bit and exchanged the modem for a more expensive one with built-in Wi-Fi. Happy end...

Answer 1

Was it right to turn it off on the router, and not on the modem?

Yes
connected correctly.
The problem was that the modem was giving out the correct settings, and the router was giving out the wrong ones, in short. Devices could pick up either one or the other, and, accordingly, they either saw the Internet or not. You turned off the source of the problem.

Answer 2

Set up an ADSL router with a bridge. Make PPPoE, DHCP, Wi-Fi settings on the Wi-Fi router and start living. These actions will simplify configuration and remove controversial points, stability will increase.
Or sell an ADSL router and a Wi-Fi router and buy an ADSL router with Wi-Fi , then it will become completely good.