Not able to connect to 5Ghz WiFi network on my iPad.

It's totally your call. To me, just because it worked, doesn't mean it will continue to do so ... as you proved. I'm sorry, I really can't assist you any further until you can verify that your iPad can still connect to other 5 GHz Wi-Fi networks. If you don't want to try my suggestion, then hopefully, someone else will chime in to offer you other alternative options.

The bottom line is that we don't know for sure if this iPad is the issue or not. Good luck!

Well, my suggestion is one way to verify, whether or not, your new wireless router is the issue and not the iPad.

Something else you can try is to reset your iPad's Network Settings to see if this will resolve it with your router:

• Tap on Settings > General > Reset > Reset Network Settings

Per chance did you configure your new wireless router to "hide" your 5 GHz network? That would be one reason that it doesn't appear in the iPad. If this is the case, you would have to choose the other networks option, and then, enter the name and wireless password for your network.

If you didn't hide it, then you could try other resets, or even restoring your iPad all together (a bit drastic) ... OR try connecting it to another 5 GHz network as I had suggested earlier.

Can your iPad Air connect to other 5 GHz wireless network, just not this particular one?

Ref: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

Most likely you are right ... but we won't know if the old iPad model that you have is the issue or not until you can try connecting it to another 5 GHz network.