When I do a full and proper reset to defaults for a customer or when I want to be sure that firmware is truly and properly at default values:
1) Reboot the router. Wait at least 5 minutes for all startup processes to finish.
2) Create a backup of the current config and name it descriptively including the firmware version it should be used with, the router mode and the date of the backup too.
3) Save copies of any custom files created in JFFS folder (if used).
4) Save a copy of the actual firmware itself and put all of this in a single RAR (zipped) file. With WinRAR, I also add a recovery record and also test the new rar file.
With the preventative steps out of the way... let's continue
5) Unplug all USB devices from router.
6) Unplug all LAN network cables from router (unless using one to connect to the controlling computer, of course). Do not unplug the WAN cable.
7) Reboot the router.
8) Reboot the router. Wait at least 5 minutes for all startup processes to finish.
9) Flash the new firmware. Verify it is for the router you're flashing.
10) After the router has rebooted with the new firmware, do the minimum necessary to access the internet. Do not configure anything else.
11) Reboot the router. Wait at least 5 minutes for all startup processes to finish.
12) Go to the gui and do a reset to factory defaults. If it asks for a reboot when it's done, I will physically pull the power from the device. If the router reboots on it's own, I will physically pull the power two minutes after it has rebooted.
13) I will leave it without power attached, along with also turning off the ISP modem (and any other switches, AP's, NAS', printers and any other network devices, for at least 10 minutes and up to 30 minutes or more at this point.
14) Power up the ISP's modem. Wait 2-5 minutes depending on the ISP service provided.
15) Power up the router. Wait at least 5 minutes for all startup processes to finish.
16) Create new ssid's (easier than forgetting the old ssid's on all devices) for both bands.
17) Configure router with your custom requirements.
18) But: Don't change defaults for most settings, yet.
19) Reboot the router. Wait at least 5 minutes for all startup processes to finish.
20) Create a backup of the current config and name it descriptively including the firmware version it should be used with, the router mode and the date of the backup too.
21) Save copies of any custom files created in JFFS folder (if used).
22) Save a copy of the actual firmware itself and put all of this in a single RAR (zipped) file. With WinRAR, I also add a recovery record and also test the new rar file.
23) When you're ready to test these settings, do a hard reboot of the router.
With all of the above steps completed, you are ready to try tweaking the default settings to see what works best for you. I recommend at least a reboot via the gui or a hard reboot after you've made the changes you want to test.
You can now quickly go back to either your original firmware configuration or your new default firmware's configuration to quickly test and compare settings.
The above may sound like a lot, but doing it is actually easier than reading or typing it out.