Create a backup config file of your settings with a copy of the firmware that works.
Flash the A2 RMerlin firmware. Reset to factory defaults and then do a minimal and manual configuration (do not use the backup config file you created) to secure the router and connect to your ISP.
You may also want to use new SSID's too (at least to test at this point).
Test that the device connects and then continue manually customizing the router to set it up as you need checking at each step that all devices are behaving as expected for you.
If the above tests fail, simply flash the firmware you know works, do a reset to factory defaults and then restore with the backup config file you created to get you back to a working place (i.e. where you are now).
After flashing RMerlin's A2 firmware, be sure to also use the format the JFFS partition on the next boot option and then reboot the router 3 times in the next 15 minutes, waiting 5 to 10 minutes between reboots too.