it also will only connect to 2.4GHz). Everything else that I have on my 5GHz network is still workin

I've been using this router for the last several years with very few issues. A few days ago I noticed a printer I occasionally print to via wifi wouldn't work (it connects to my 2.4GHz network).Also the app for my smart thermostat quit working about the same time (it also will only connect to 2.4GHz). Everything else that I have on my 5GHz network is still working great. 

To be honest I'm not sure if I've ever upgraded the firmware on this as I never did on my previous router. I tend to be the kind of if it ain't broke type of person, lol. Anyway I just logged into my router and here are the stats I see:

Firmware Version: V1.2.0.26_1.01

2.4GHz is on channel 11

Any ideas on what's going on here and suggestions on how to fix it. Maybe I should try and upgrade the firmware first if the current version of firmware for this router is working well (and bug free)? I do live in a large subdivision with close neighbors on both sides of me. Maybe one of them changed something in their setup and the 2.4GHz channel I'm on (channel 11) has become overly crowded? Not sure how I would check any of this?

Open to any and all ideas and suggestions:)