You can use these commands through telnet if on original firmware or on the cmd page if on RMerlin's.
nvram set pci/1/1/ccode=EU
nvram set pci/2/1/ccode=EU
nvram set wl0_country_code=EU
nvram set wl1_country_code=EU
nvram set regulation_domain=EU
nvram set regulation_domain_5G=EU
nvram commit
reboot
To change back to the US just change all the EU for US.
This would change your wireless regulatory domain to Europe on both radios which has more 2.4GHz channels, less 5GHz channels and also lower TX power output than the US.
Take care
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