I am using Openwrt for a long time...
And for a long time a have a connection problem with my Archer C7 V2. (relay mode)
My Laptops Handy's etc. takes a lot of time to find my Archer. After starting my Laptop i have to wait (1-5 minutes) until the wlan of my laptop will see the Access Point of my Archer C7. All other's SSID's (neighbours) will appear immediate.
May be, i have to set a specific parameter ?
Are you using some channel which is not usually allowed in the country? like 14 in 2,4GHz?
Are the channels that the neighbours use overlapping with your?
There is a setting for beacon interval but this should be every 1000msec by default, if I am not mistaken.
Have you tried to scan the spectrum with the mobile phone apps Net Analyzer or Wifianalyzer?