Are you having any adverse effects at all from this or is it just simply an annoyance as you have stated? Also, have you shut down all devices connected to the router and checked to see if the number still fluctuates? Have you looked for any traffic that may be unauthorized?
Any time I see adverse effects like this my first go to is to ensure something is not being exploited. This is hard to do though if your not familiar with those types of attacks. Where as this seems almost software/firmware related it may be prudent to take time to perform a few safety steps. Though this particular symptom could be indicative of a DeAuth/Poisoning attack that I would use for pen testing.
1. Change your SSID
2. Change your password. (Hint: Simple passwords on routers is akin to closing your door and hoping a thief thinks its locked, all routers can be exploited)
3. Hide your SSID. (This will only stop the unskilled exploiters)
4. Check if the issue still occurs.
5. If issue is still recurring after changing above steps weigh the below options at risk level/confidence.
6. Hard Reset
7. Rollback firmware.
8. Dynamic/Static IP Assignments.
9. MAC Filter (Always use this feature unless your constantly having quests with different equipment)(https://www.microcenter.com/tech_cen...ireless-router
10. Lock mobile devices to a longer range band. (IE. Network 802.11G ~ 2.4GHz) (Remember, as you increase throughput speeds you reduce range)
11. Verify devices on the 802.11n/ac network band are in sufficient range not to have connection issues. (This could be fallout from MIMO antennas constantly opening new channels and breaking them again)