Spotpass is still working (somehow!) despite the eShop closure - I don't think ninty care enough to hand out bans for modded 3DS's anymore - but I may be wrong.
Go to Streetpass Mii Plaza settings > Turn on Spotpass > put your console in sleep mode for a while
It took me a few tries and turning the setting on and off again and putting into sleep mode for each instalment, but i got there eventually, and with the help of the save editor I now have all puzzles completed
I love being able to help people complete their puzzles in a time when the software has been all but abandoned everywhere else - Thank you for your service