I looked at several solutions for this question, but none of them worked. Additionally, I tried some of the tutorials provided (with the necessary changes), but none of them worked

My aim is to use a RPi 3B+ as a router with an existing wireless router connected as an access point. The following figure explains this:

'eth0' is the Rpi's Ethernet port. The IP for eth0 is assigned automatically by a DHCP server that is beyond my control.

'eth1' is an 'USB to Ethernet' adapter. The wireless network is separated from the network the switch is in by a NAT service that should run on the RPi.

I am following this tutorial as a guide. I have already configured the router as an AP.

My Problem: When I configure the network (i.e. edit the /etc/network/interfaces file) and restart the networking services (sudo /etc/init.d/networking restart), I get the following error:

Restarting networking (via systemctl): networking.serviceJob for networking.service failed because the control process exited with error code. 
See 'systemctl status networking.service' and 'journalctl -xn' for details. 

I also get a similar error when I try to start the DHCP server after configuration.

Starting isc-dhcp-server (via systemctl): isc-dhcp-server.serviceJob for isc-dhcp-server.service failed because the control process exited with error code. 
See 'systemctl status isc-dhcp-server.service' and 'journalctl -xn' for details.

In addition, when I install isc-dhcp-server using sudo apt-get install isc-dhcp-server, it gives an error saying failed to start although it installs properly.

Can someone show me what I'm doing wrong?

EDIT: I have made a change to the setup. Instead of getting a direct connection from the ISP, I get it through a switch (hence no modem). What happens beyond the switch is none of my concern)