Inclusion Criteria:  Provision of signed and dated, written informed consent prior to any study-specific procedures, sampling, and analyses. Aged at least 18 years as of the date of consent. Histological or cytological confirmation of a solid tumor, and has progressed despite standard therapy, or is intolerant to standard therapy, or has a tumor for which no standard therapy exists or for which standard therapy is not considered adequate. Estimated life expectancy of a minimum of 12 weeks. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1. Female and male subjects should be using adequate contraceptive measures as requested.  Exclusion Criteria:  Subjects with CNS tumors or known CNS metastases will be excluded. Prior ATG-101 administration or a 4-1BB agonist. Prior anti-tumor systemic therapy within 21 days(a period of 5 'half- lives') of the first dose of study treatment. Radiotherapy with a wide field of radiation within 28 days. With the exception of alopecia, any unresolved toxicities from prior therapy greater than Grade 1 (CTCAE v5.0) at the time of ICF signature. Active infection, including hepatitis B and/or hepatitis C. Have uncontrolled intercurrent illness, including but not limited to: Inadequate bone marrow reserve or organ function. History of hypersensitivity or history of allergic reactions attributed to drugs with a similar chemical or biologic structure or class to ATG-101. Prior organ allograft transplantations. Pregnant or nursing females. Have a history of another primary malignancy within 3 years prior to starting study treatment. Exceptions are as follows: the disease under study; adequately treated basal or squamous cell carcinoma of the skin; cancer of the cervix in situ, etc. In the opinion of the investigator, subject's complications or other conditions may affect protocol compliance or may be unsuitable for participation in the study.