DHCP advice needed for 4G modem to own router

I am a bit out of my depth setting up my 4G modem to integrate with my wireless router and would really appreciate some advice.

The equipment I am replacing is the usual FTTC modem in bridge mode through to my ASUS AC86U which then goes on to my (non-PoE) 16-port switch.

I now want to use an external antenna, external 4G modem (Category-6 modem, 2.4GHz wifi and own DHCP) running OpenWRT using a PoE injector and of course my ASUS AC86U to the switch.

The issue is that I don't understand the DHCP settings that I needed for these devices.

At the moment the 4G modem is set up as 192.168.1.1 / 255.255.255.0, static address. DHCP is enabled and issues address in the range 100 to 150. IPv6 is currently not enabled.

Because it has been connected to the FTTC modem, the ASUS AC86U is currently set to PPPoE with DHCP running and has a static address of 192.168.1.254. Its own pool is 2 to 253 and there is one static address assigned to a NAS box of 192.168.1.202. All other devices are dynamic addresses (wired desktop PC, Mac Mini, AppleTV, MVHR network card. Wireless iPhone, iPad, HomePod).

Which device should be running the DHCP server? What address pools should I be allocating?

The final piece of the puzzle is that for a while I may want to use my old BT FTTC connection in dual-WAN so assume it needs an address different from its current 192.168.1.1, yes?

Only one router should be running a DHCP server, you have to decide which router and put the other router into bridge mode so it act essentially like a dumb switch.

Which one is the server depends on the functionality you need and the router that ticks the most boxes, some will not even run bridge so the decision is already made.

I run a main router and have 4 other routers bridged into it, for ease of maintaining them I assign all the bridged routers fixed IP's within the main routers net mask but outside of the DHCP range. I can then access their setup pages with their fixed ip address.

Thank you that helps.

I am in the process of returning the external modem and antenna as its performance was not worth the price premium.

This leaves me with the B525-23a that EE are currently handing out so I would like to put it in Modem Mode and then use my ASUS AC86U as the main router but I don't know the PPPoE settings that I need on the router. Is there a way I can find out what I need to enter on the router to do this?

The B525 will not support PPPOE, set the B525 to be the DHCP server, run a cat5 from lan port on B525 to wan port on asus and set the Asus up to collect an IP address from the B525.