This is likely IT101 for experts, and better methods likely exist, but after tricking a google search to not yield garbage and actually give me the answer... Namely, how to get multiple routers to let a PC connected to one of the two routers in the house see shared files on a PC on the other router (if you got sharing files to work on PCs all connected to one router, now it should work over these multiple routers once you do this trick).
One of the routers is connected to the cable modem, which I'll call the primary router. It has the usual IP address 192.168.1.1, and has DCHP enabled to assign IP addresses from 192.168.1.100 to 149. Now here is the trick with the other router, I'll call it the secondary. With the secondary router go into its configuration web page (usually you connect a PC with a web browser to a LAN connector and open its default IP address (with nothing else connected other than the power right now), usually 192.168.1.1 (check the manual) and do the name and password thing. Okay, once in, change the IP address to something in the same range of the primary's IP address, if the primary is 192.168.1.1 use say 192.168.1.XX where XX is a number between 10 and 99 (to avoid intruding on the primary's DCHP IP address assignments). Also disable the secondary's DCHP, and maybe also disable the secondary's NAT. After saving these settings you'll need to use the new IP address to log back into it again, so disable the DCHP and save it and then change the IP address.

Now connect a CAT5 cable to one of the secondary's LAN connectors and a LAN on the primary router. Nothing connects to the secondary's WAN connector. What seems to happen with the secondary router is that its LAN ports look to behave like they were additional LAN ports on the primary router, and computers connected to it will look like they are connected directly to the primary router, and the primary's DCHP does the assignment of IP ddresss. Say the primary has 13 LAN ports, and the secondary has 8 LAN ports, now it looks like the primary has (13-1)+(8-1)=19 LAN ports (the -1's represent the LAN to LAN connection between the routers). Yeah, I ran CAT5 all over the house so I can jack in my PC wherever I go (wifi is way too congested in this neighbourhood). And now I can share files now that I found and did the above trick

Here the secondary router is between the cable modem and the primary. I used the LAN ports that have the contacts on the bottom first, to avoid those contacts getting dirty from dust. I used screws to reinforce the wooden crate to be sure it doesn't fall apart with all this stuff in it.

What you are doing is wiring your secondary router to act just as a hub (or switch) on the primary network. The "router" functionality is irrelevant in that usage. You could do the same thing with any commodity switch or hub. I often see 16-32port 100Mb/s switches discarded at the local rubbish tip from office installations that have upgraded to gigabit LANs. You can also daisy-chain any number of 4-8port switchesif you have any lying about, up to the 256-port maximum on your main router's DHCP.
LAN configurations are actually quite easy to extend. Problems are only likely to arise if you have some devices on the LAN set up with fixed IP addresses that might conflict with the DHCP address allocation.

Maven

Interesting re-use of bread tags.

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A valve a day keeps the transistor away...

Yes, a box made for being a hub or switch would be the choice if buying new, but as I already had the second router, I made it act as a hub or switch. Conflicts with DCHP vs fixed IP addresses shouldn't happen, as anything with a fixed IP address (I'm not sure if the LAN device IP address I tell a router to be counts here) is not in the range the DCHP will assign, namely 100 to 149.

QUOTE: What you are doing is wiring your secondary router to act just as a hub (or switch) on the primary network. The "router" functionality is irrelevant in that usage. You could do the same thing with any commodity switch or hub. I often see 16-32port 100Mb/s switches discarded at the local rubbish tip from office installations that have upgraded to gigabit LANs.

Last weekend, at a garage sale, I picked up a 24 port FS728TS Netgear switch, $15. Possibly this switch came out of some office installation that upgraded as you mentioned.

I also had to build a wider "rack" to hold it, along with the RV016 router (configured as a router with enabled DHCP, and my ISP's cable modem. All running off of 240V60Hz. More ports (12 + 23 = 35) than I'll ever need.

Good catch! In my experience the Netgear pro products are generally quite robust, well shielded and with good heat management. If anything fails it's usually the power supply, easily replaced.

Maven