If you want to use your router as just a straight modem, ie turning it into just a modem, that means connecting it to your router and getting the router to speak PPPoEoE to it. People sometimes refer to it as ‘bridged mode’. I just want to confirm that that’s what we’re talking about.
This is what I do with my modems. I have ZyXEL modem-routers and have them configured to be in straight modem-only mode.
Now in this case, dhcp server function on the device doesn’t mean anything, as it isn’t bring a router any more and acting as a dhcp server to PCs etc on the LAN, handing out in addresses etc, is not something that it will be doing any more. Being a dhcp server for PCs etc is now a job for your main, real router (or some other dhcp server) on your main lan.
If ‘dhcp’ is still enabled on the now-modem then if that means dhcp server then it won’t be doing anything.
If (really unlikely) it means ‘be a dhcp client’, then meaning ‘get myself an IP address from some dhcp server somewhere’ then that might or might not be quite useful, as we need to decide somehow on an IP address for the admin function of the LAN i/f of the modem if we want to talk to it, use its web UI, get stats from it or whatever. We might have to use two Ethernet cables, one to get stats and do admin, and one for the user data connection to the router via PPPoEoE, if the router is going to prevent your talking through it to get to the modem.
Just to get data in and out of the modem to and from the internet via the router you do not need an IP address for the modem, because pppoe doesn’t use IP at all only lower level Ethernet stuff.
However, as I said, if you want to administer it, you will have to decide on an IP address for the admin interface, if not, then it doesn’t matter. There has been a lot of discussion about getting admin and stats access set up in other threads and articles here.
I had a lot of brain ache with my router working out how to get access to the modem _through_ it, as I wasn’t sufficiently familiar with the advanced features it has and I also chose ip addresses for the modem’s admin function which made things awkward for router setup (and for modem setup too). But I have four modems so life was likely to be hard.