Inclusion Criteria:  Ability to understand and the willingness to sign a written informed consent form. Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1. At least one measurable lesion is required (RECIST v1.1) Phase 1b: Subjects with pathologically or cytologically confirmed recurrent or metastatic NSCLC without known EGFR mutation and ALK and ROS1 gene rearrangements. Phase II: Subjects with pathologically or cytologically confirmed recurrent or metastatic malignant thoracic tumours who have received 1-2 lines of systemic anti-tumour therapy, including platinum regimens, and have failed, including at least 2 cycles of chemotherapy.  Exclusion Criteria:  Prior exposure to 4-1BB agonists. Receipt of any anticancer investigational product or any approved drug(s) or biological products (except hormone-replacement therapy, testosterone or oral contraceptives) within 4 weeks prior to the first dose of study drug. Previous exposure to oral fluorouracils or small molecular targeted drugs require a minimum washout period of 2 weeks or 5 half-lives prior to the first dose of study drug (whichever is longer). Previous exposure to mitomycin C or nitrosourea requires a minimum washout period of 6 weeks prior to the first dose of study drug. Receipt of PD-L1 therapy within 24 weeks prior to the first dose of study drug. Known allergies to CHO-produced antibodies, which in the opinion of the Investigator suggests an increased potential for an adverse hypersensitivity to ES101. Grade ≥ 3 immune-related adverse events (irAEs) or irAE that lead to discontinuation of prior immunotherapy. Some exceptions as defined per protocol apply. Subject has not recovered from all AEs of previous anticancer therapies to baseline or ≤ Grade 1 per CTCAE v5.0 before teh first dose of study drug. Certain exceptions as defined in protocol apply. Active autoimmune disease or documented history of autoimmune disease that required systemic steroids or other immunosuppressive medications. Certain exceptions as defined in protocol apply. Active interstitial lung disease (ILD) or pneumonitis or a history of ILD or pneumonitis requiring treatment with steroids or other immunosuppressive medications. Systemic anti-infectious drug treatments within 4 weeks prior to the first dose of study drug. Pregnant or nursing females.