Server running Ubuntu 16.04, 2 NICs via Technicolor TC8305C; causing Inteet issues w/ other clients when Port Forwarding / or DMZ enabled?

Just for completion purpose, here's the info in detail again (as is in title):

• I'm running Ubuntu 16.04 on server hardware;
• 2 x NICs (enp4s0f0 (AKA first), enp4s0f1 (AKA second); 10.0.0.10, 10.0.0.11 respectively) are connected to Technicolor TC8305C wireless router/gateway (rented from and connected to Comcast cable Inteet)
• I need following ports to be reached via the Inteet (to run a 7D2D game server): 26600 (TCP & UDP), 26601 - 26603 (UDP) and 27000 - 27099 (UDP) to one of the server's NIC at IP 10.0.0.10

I have several networking issues occurring:

1. First of all, when I d/c the second NIC, configured to 10.0.0.11, I can still ping that IP via my desktop PC (connected via WIFI through same gateway)... this leaves only one explanation for me, that something's messed up with the network settings on the server but reading up on this for two days now I have not been able to fix this.

2. My wife's and my Windows PCs connected via WIFI to the TC8305C wireless gateway have problems to connect to the Inteet from time to time (it'll work for a few seconds, then it'll stop for perhaps the same amount of time); I run now a ping -t mail.google.com from my PC and I see for a while "Request timed out." and then for a while the ping works, and back and forth.

3. When I deactivate port-forwarding to 10.0.0.10 and/or the DMZ, after a restart, the issue sometimes stops; reactivating either one, the connection issues from the Windows PCs come back - sometime I have to factory-reset the gateway for the issue to go away.

4. Disconnecting the two NW cables from the router/gateway, I did a factory reset and reconfiguration, this time activating only DMZ and connecting only the cable to the NIC assigned 10.0.0.10, the issue immediately comes back.

5. No connection issues to the server from my Windows PC (WIFI) and no inteet connection problems from the server itself.

/etc/network/interfaces:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto enp4s0f0
iface enp4s0f0 inet static
 metric 0
 address 10.0.0.10
 netmask 255.255.255.0
 gateway 10.0.0.1
 dns-nameservers 8.8.8.8
 dns-nameservers 8.8.4.4
 up /usr/sbin/wondershaper enp4s0f0 5000 1000
 down /usr/sbin/wondershaper clear enp4s0f0

auto enp4s0f1
iface enp4s0f1 inet static
 metric 1
 address 10.0.0.11
 netmask 255.255.255.0
 gateway 0.0.0.0
 dns-nameservers 0.0.0.0
 up /usr/sbin/wondershaper clear enp4s0f1
 down /usr/sbin/wondershaper clear enp4s0f1

After disconnecting both NW cables from the router/gateway, I'm still having same connection issues and only factory resetting it solves the issues (and not connecting any and all of the server network cables).

Also to note, this issue started after I did a server update and restart, at least that's how it seems to me and from what I can remember.

Any commands I should issue on the server to post here that'd help figuring out the issues, let me know and I'll paste the results here.

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