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It is difficult to say with certainty without reading the clause itself.
The lease represents a contract between you and the landlord. Contractually the landlord can request you take the sign down if there is a clause in the lease prohibits signs being displayed.
The fact you own the apartment does not matter as the terms of ownership are set out in the lease. If displaying signs are prohibited then the landlord can insist the sign be removed. I would be happy to review the clause in order to give you a certain answer.
Sadly, I do not think you can raise any human rights issue here.
I hope this helps.