I have checked. The requested feature was supported. Please upgrade to this firmware - http://download.peplink.com/firmware/br1/fw-max_br1_br2-6.3.3s047-build2342.bin
Below is the screenshots I tested with this firmware.
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I purposely enter an invalid USSD code
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I enter a valid USSD code
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I choose 1
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I choose 0
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Back to the main menu as expected