It is the US administration, among other signatories, that certifies compliance. Acording to US Law, the President is in fact required to do so every 90 days:
Per the Iran Nuclear Review Act of 2015, the Trump administration is required to certify to Congress every 90 days that Iran is in compliance with the July 2015 nuclear deal (the Joint Comprehensive Plan of Action, or JCPOA) and that this agreement is in the national-security interests of the United States.
The IAEA, as claimed in the accepted answer, does not certify compliance
These accounts are false. In fact, the IAEA has never certified Iran’s compliance with the Joint Comprehensive Plan
of Action (JCPOA).
Rather, the IAEA’s mandate with respect to the JCPOA primarily entails monitoring and reporting on Tehran’s
nuclear-related actions (or lack thereof) pursuant to the JCPOA’s provisions. The determination of whether Iranian
conduct constitutes compliance with the JCPOA remains the prerogative of the individual parties to the agreement:
China, France, Germany, Russia, the United Kingdom, the United States, and Iran, with the high representative of
the European Union for foreign affairs and security policy
...has proclaimed it will not permit inspections of its military bases, which are permitted—indeed necessary—under the nuclear deal.
In prepared remarks on deciding to decertify Iran, Trump said
Iran failed to meet expectations in its operation of advanced centrifuges and intimidated international inspectors into not using their full authority.
There is also significant cause to believe that the IAEA has been severely compromised and is not acting dutifully to their mandate:
It is also necessary to investigate the conduct of the IAEA, which was obligated to act an independent professional body not subject to political pressure, but wrote its reports of inspections in consultation with the party under investigation, that is, Iran. The conduct of IAEA director-general Amano must be examined, since he agreed to a scandalous, unprofessional, and unreliable inspection procedure at the Parchin military site; also there must be an examination of the IAEA's claim that it was impossible to determine with certainty that the suspect activity was carried out by the Iranian regime.
The Trump administration duly decertified Iran