That router is likely 5 GHz capable, especially since it is modern enough to feature WPA2 encryption.
Best practice has always been to use "automatic" channel section (inside each 2.4 GHz or 5 Ghz band). When set to automatic, your Router listens to the existing traffic as it starts up, and chooses the least-busy channel. It is not an accident that every debugging procedure starts with "cycle the power to your Router."
Be sure to set EACH band to "automatic" channel selection.
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The Mac will try to connect automatically on what APPEARS to be the "best" channel, and your manually-assigned channel on 2.4 GHz looks great... until your neighbors start using a nearby channel and clobber your data.
if that does not help enough, you can try is to 'split' your network and give each band its own network-name. Then you can selectively connect to either the 2.4 GHz (by one name)or the 5 GHz (by a slightly different name) because each will have a unique name. This also requires a change inside your Router.