Cisco Router Does Not Pass Traffic to Next Hop (Internet Router)

We have an old Cisco 1800 in a test lab.  The two network interface are configured as follows:

FastEthernet 0/0

FastEthernet 0/1

The default gateway for PCs on the 6.X network is 6.33 - that is how they can get to the 5.X network.
The default gateway for anyone on the 5.X network is 5.1 (another router with Internet access).
The router with Internet access (5.1) has a static route so it can pass traffic to 6.X from 5.x via (interface on the Cisco 1800).
All systems on the 5.X network can get to Internet no problem because their gateway is 5.1 and it can either send traffic to Internet or back to Cisco for access to 6.x network.
PROBLEM - All systems on the 6.X network CANNOT get past the 5.X network when trying to reach Internet.

Question #1 - How can we tell the Cisco router that traffic coming from 6.X needs to get to the Internet via the router on 5.1?
Question #2 - Do we need tell the router on 5.1 (a Sophos UTM, not cisco) anything about the 6.X network that is not directly connected to it?

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Jacob DurhamIT Support Analyst II (Lead Infrastructure Engineer)Commented:
Can you give of the output of

show ip route

Open in new window

from the 6.x network?
dpmoneyAuthor Commented:
show ip route

Gateway of last resort is to network

C is directly connected, FastEthernet0/0
C is directly connected, FastEthernet0/1
S [1/0] via
S* [1/0] via
Jacob DurhamIT Support Analyst II (Lead Infrastructure Engineer)Commented:
Is this what your network looks like? What are the other interfaces?

dpmoneyAuthor Commented:
Not exactly, but we just figured it out by adding a firewall rule on the upstream router (the Sophos UTM).  That appeared to be the issue.  Thanks for your time, nonetheless.
dpmoneyAuthor Commented:
Closing question per previous comment - thanks!

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