My suggestion is to try each channel available for your router and client. Test not just for maximum download speed for both ISP and local LAN performance, but also for the lowest latency, the smoothest graphing of that download and the most responsive web browsing you're able to achieve. The balance of all these will give you the best overall experience.
You don't mention what router, firmware, channel width or other information that may be pertinent to this discussion.
But consider using the latest firmware (RMerlin and the RMerlin forks; john9527 and hggomes for certain Asus routers and even the RMerlin based XVortex fork for the R7000 too).
In addition, if moving to a new firmware or if you haven't done this already; reset the router to factory defaults and manually and minimally configure the router to secure it and connect to your ISP. Do not load a saved config file, but you might want to try john9527's nvram save/restore utility (again, on Asus routers) to save yourself a few minutes typing things in manually (or at the minimum, having a complete text based listing of all the settings that you have now).
I would also use new ssid's for both bands or 'forget' and re-associate on each device you connect to the router.
Especially if you flash new firmware, do not automatically assume that the old settings are the optimum settings. Each change you make from the defaults (defaults which may change in each new firmware) deserves to be tested individually from the start to determine if they are still beneficial.
For example, on the latest RMerlin firmware and forks I only change the Preamble to Short for both bands, use a specific Control Channel (and only 1, 6 or 11 for the 2.4GHz band) and use 20MHz channel width for the 2.4GHz band and 20/40/80 MHz channel width for the 5GHz band.
Physically raising your router to at least 10' above ground level and also locating it closer to the area to be covered wirelessly will also help. Make sure to leave at least a couple of feet of space behind and above it too.
Hope to see that one or more of these suggestions help. Report back whether they do or not.