I have produced some extensive documentation of my attempts of getting a Raspberry Pi 4 4GB running the most Raspberry Pi OS 64-bit (Debian bullseye arm64) and a WaveShare SIM8200EA M2 5G HAT modem working as a residential gateway combination. I got the modem working and able to connect to the internet on the Raspberry Pi 4 4 GB locally, but the packet forwarding seems to not work at all. When tracerouting the packets, their path always ends at my Raspberry Pi 4 4 GB supposed-to-be residential gateway without being forwarded to the ISP-provided dynamic IP address and tiny network. I have made a summary thread on the Raspberry Pi Forum containing firewall configuration, /etc/sysctl.conf settings, ISC DHCP server configuration and routing tables:

https://forums.raspberrypi.com/viewtopic.php?t=346017

I have the following network configuration:

    1) a private network 192.168.1.0/24

    2) inside 192.168.1.0/24 an OpenWrt operated ethernet PoE+ DSA switch

    3) inside 192.168.1.0/24 an OpenWrt operated Wifi router which is connected via its WAN interface to the ISP provided modem on 192.168.0.0/24 network. The ISP modem does not allow anything, it is basically an intransparent bridge. The Wifi router is running a DHCP server on its LAN interface containing the LAN ports and it has another DHCP server running for managing the Wifi connections.

    4) my Raspberry Pi 4 4GB / WaveShare SIM8200EA-M2 5G HAT combination running with a temporary SIM card, till i can replace the ISP provided modem by it. The RPi/5G HAT has lo, eth0, wlan0, wwan0 (plus usb0 from thethering via dwc2 and g_ether) interfaces and could be used as DMZ as such. The RPi/5G HAT eth0 is connected to the Switch and has IP 192.168.1.1, which should be the residential router/gateway IP. wlan0 is connected to the Wifi router. I have configured a working nftables firewall filtering ICMP traffic for both IPv4 and IPv6 that works nicely when i use the internet access locally on my RPi / 5G HAT. wwan0 is the 5G HAT modem interface, which is configured via NetworkManager (is it possible also via /etc/network/interfaces?) and ModemManager and has a small private subnet assigned by DHCP from the ISP (my ISP also allows for public dynamic IPs which are routable in the internet). I have configured a static route from the RPI / 5G HAT to the Wifi router, such that it can have internet access via the ISP modem, even when the 5G HAT is turned off.

Now how can i get my RPi / 5G HAT to become a residential gateway to serve internet access to the whole 192.168.1.0/24 home network including the Wifi devices? When i install an ISC DHCP server on my RPi / 5G HAT, it always messes up with the DHCP server of the Wifi router and the Wifi devices loose internet connectivity. As far as i have understood, routing functionality in internal networks is provided via DHCP and DNS server's as well as activation of ip forwarding.

(Taken from https://forums.raspberrypi.com/viewtopic.php?t=346014)