turning an embedded linux system into a router LAN/WLAN/WWAN

Question

I am attempting to establish communication among a network of embedded Linux devices over a small local network. One of the devices, serving as the DHCP server, should have an interface with the broader internet.

I have limited knowledge about networking, which I believe is also impeding my progress in this process.

possible configurations:

• receive signal via 4G and bridge this to the wifi hotspot/ethernet port
• receive signal via the ethernet port and bridge this to the wifi hotspot

what I have working so far:

• ethernet dhcp server
• wifi hotspot
• 4G connection
• wifi connection
• ethernet connection

but not the bridges between them.

system details:
kernel 5.10.72
debian 11 bullseye
arm64/aarch64 architecture
using network manager and dnsmasq

i tried this: How to create wireless bridge connection with nmcli

but the bridge was unable to be activated.

I'm really not quite sure what direction to look in, there seem to be so many available ways to do this but I can't get any of them to work, brctl, ip.

So far what I see is that network manager bridges connections together and not interfaces

I tried:
ip link add name wifi-eth type bridge

but this gives me:
Error: Unknown device type.

I tried:
brctl addbr wifi-eth

But this gives me:
add bridge failed: Package not installed

Does anyone have some advice for me, maybe just resources that can help me?