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When you first create an account it is PPLNS
Help->Solo explains the requirements to change an account to Solo
If your account is stuck on PPLNS, then yep you missed the requirements written on Help->Solo
In that case, after reading Help->Solo, create another account (with a different BTC payout address) and switch that to solo.
I changed to solo
But it shows pplns on my site
2 pplns 9bb5qzleu3ix2 3,965.44THs
Sorry, nope, your account is pplns - it didn't change to solo.
The web site tells you that it failed to change to solo and why it failed.
You missed the line on Help->Solo that says you can't change an account after mining on it in PPLNS.
Edit: also, when you login, it says in bold
red on the top left what type of account you have.
I have 50 m30 devices. How much should I set the difficulty number?؟