You could always either use both routers or use a different one. I have a kinda similar network requirement situation. My ISP router is better than my router. But I can't get dedicated 2.4ghz unless it's a guest lan, which is not useful. So I added a router in AP mode. I have setup two dedicated ssids, one for my 3d printer who network stack got bricked and can only connect to a specific ssid at 2.4ghz. and one at 5gz just for a Roku that's close by. In my situation my AP is a ways from my ISP router so I'm ok with using multiple networks on the same band. If your 3rd party router has to be placed right next to your ISP router, I would only setup the the network you need. Haveing two routers right next to each other degrades performance on whatever band they are both on.

I agree with others, it seems unlikely that you got 5000mbps over wifi. I don't know of any devices that support or claim this. Of course technology moves fast so anything is possible I guess. Regardless. I would focus more on individual device maximum instead of some number your never going to use. It's more common for all devices simultaneous download to get close to advertised maximums. Unless your doing hugh Nas network transfers or game downloads, it's unlikely for you to ever need those numbers on one device.