Thank you very much Glitch, alzhao, and RangerZ for all your input. It was all quite helpful and an excellent learning experience for me. I tried all three methods –

Method 1: Uploading two scripts via winscp:

I taught myself how to use winscp and logged into the router successfully using ‘root’ as username, with regular password and scp as file protocol, found the respective directories (/usr/bin/ and /etc/rc.button/), uploaded the scripts (switch_mode and BTN_1) into their respective directories, and in each case then went into the command line while in the respective directory and ran the chmod +x command to make the files executable.

I could not get this method to work. I think perhaps I wasn’t using issuing the chmod +x command correctly because nothing seemed to change at all in the behavior of the MT300N. Even if I moved the physical switch to the right, the MT300N stayed the same and still allowed me to log into it at 192.168.8.1 and it was not bridging to my main router’s network. I rebooted the router, checked that the scripts were indeed there via winscp (they were, and were check marked executable), but nothing changed. Internet worked, but I was not bridged to my main router’s network.

Method 2: Using Advanced Settings (Luci) to accomplish the same as Method 1 but with no physical switch:

This method worked perfectly! I went into Luci, followed alzhao’s instructions, and 10 seconds later I was receiving internet and could see my main network. The wifi signal from the MT300N is strong and is definitely bridging back to the main router via the Ethernet connection between the two routers. All apps and devices upstairs on the new SSID are seeing the main network flawlessly. Mission accomplished. (Though, as expected, I can no longer log into the MT300N.)

Method 3 – Using Luci, configure Dumb AP via steps on wiki.openwrt.org:

I could not get this method to work. Using Luci, I followed the steps and set an IP for the MT300N right next to my main router’s IP, then disabled DHCP, then disabled Firewall. But this simply broke the MT300N and broke all internet. The help doc said that I would be able to re-access the MT300N via the new IP I had specified for it, but this did not work. Not sure what I did wrong here.

In particular, I would like to still try and get Method 1 to work. All is working perfect now with Method 2, but it would be a nice elegant solution if I had a physical switch to switch the MT300N between regular router operation and AP/Bridge. That way, I could log back into the MT300N anytime I wanted by simply moving the switch to the left. I suspect I messed up the ‘chmod +x’ command because it just seemed like the new scripts weren’t executing.

Any suggestions/thoughts on getting Method 1 to work?