Home network issues

Hey everyone.
I’ve run into an issue with my Denon 2113 receiver. I run a coffee shop where this unit is hardwired via Ethernet to the main router for internet radio. The cafe's guest Wi-Fi runs off that same router.
Up until now, I've been using a mobile app to remote control the Denon—web control, I think it's called.
Everything was fine until some local teenagers realized they could download the same app and start messing with my settings.
Is there any way to fix this without having to shell out money for new gear?
I’d like to keep my remote access, but ideally, just for me. Heh.

Unless that mobile app has some kind of user/password authorization setup...
If you're just using those standard "home" routers you get from providers like Comcast, you're probably stuck unless you want to invest. Honestly? It’s a little risky to be running your home connection, your business connection, and an open Wi-Fi for the cafe all on one "dumb" device (which is what you've got, if I'm following you right).
I think for your own peace of mind—and for the sake of your business, since this cafe actually brings in money and shouldn't be costing you a fortune—the smartest move is to grab a Cisco or something similar. Have someone set up separate networks on it for you. It'll cost you maybe a few hundred bucks, and everyone does it...

Could someone help me out with a dual-band router?

It’s not really about the brands, honestly. The first thing that jumps to mind is just finding a device that can spin up two separate networks—even if they're just virtual ones—so you can set unique authorization for each. If a dual-band setup handles that, great, but certain models from Cisco or Ubiquiti can actually pull that off on a single frequency too. The catch? You really need to know how to program them, though you can always pay someone to handle the heavy lifting. I know plenty of local coffee shops pick those up specifically because they offer that level of granular control, whereas your typical "off-the-shelf" consumer gear is pretty stingy with the settings.
The bottom line is that a dual-band setup with a wider range usually means a faster connection, and it'll likely face less interference from standard devices nearby. That said, more frequencies mean more chances to run into "noise," which can weaken the signal sooner. You can't really guess this stuff blindly, right? You know your space better than anyone, so the smartest move would be to have a pro come out and take a look in person. It beats wasting money on hardware that doesn't fit your specific situation.

Another option is to hook up a used router from an ISP to your main network and just use it as a dedicated guest access point. That way, everything else stays tucked away behind your own password on your original network. Just wait for a pro to confirm if that's a good call for your setup, since I haven't personally tried it...

I’m a bit worried that adding another router to my current setup won't actually achieve anything. If they just connect to the new one but still have access to the old network, they'll be able to reach the Denon again automatically.
I did find a router at a pretty decent price—which, mind you, is coming from someone who is a total layman—that I think might solve this. Since you clearly know your stuff better than I do, would you mind taking a look?
TPLINK TL-WDR3500
Anyway, thanks for all the help so far.

So, from what I can see, you get separate settings for both the 2.4 and 5 GHz bands. You basically just decide which one becomes the guest network and whether those guests get access to your local network or not... and that's pretty much it!