Is the lease agreement with my father, as an individual, also the trustee at the time of signing the lease, a valid lease agreement with the trust?

My brother and I are beneficiaries and current court appointed co-trustees of our grandparents revocable living trust. My grandparents were not trustees, only trustors. My father was the only trustee until a few years ago when we finally found out about the trust and made him resign after we found that he was using the rental income from the trust property for his personal use and placing it into his persanal bank account. The problem now is that a commercial tenant has a lease agreement with my dad as an individual, with no mention of the trust, in the lease agreement. My father signed the lease as an individual and not as trustee. As a result, the tenant has been paying the rent directly to our dad in his personal name and not the trust, the legal owner of the property. The tenant refuses to recognize the trust after we have sent numerous letters directing them to send the checks to the trustees in the name of the trust. My question is, is this lease valid? The terms of the lease are 50 years with a 50 year option. The property was owned by the trust many years prior to the lease agreement being signed.