Study to Evaluate the Efficacy and Safety of [177Lu]Lu-DOTA-TATE in Patients With Grade 1 and Grade 2 Advanced GEP-NET

Inclusion Criteria: * Presence of metastasized or locally advanced, unresectable (curative intent), histologically proven, well differentiated Grade 1 or Grade 2 (Ki-67 \<10%) gastroenteropancreatic neuroendocrine tumor (GEP-NET) diagnosed within 6 months prior to screening. * Participants with high disease burden in the Investigator's opinion. Following criteria should be used as the guiding principle for determining high disease burden: * Primary tumor or a metastatic lesion \> 4 cm * More than one tumor or metastatic lesions measuring \> 2 cm * Elevated alkaline phosphatase \> 2.5 X upper limit of normal (ULN) * Presence of bone metastasis * Presence of peritoneal metastasis * Symptoms due to tumor volume such as pain, fatigue, weight loss, anorexia etc. * Symptoms due to hormone excess requiring active management * Additionally, participants who, in the Investigator's opinion, have high disease burden due to their disease characteristics not specified above could also be considered eligible. * Participants ≥ 12 years of age. * RLI somatostatin receptor (SSTR) uptake on all target lesions (defined by RECIST v1.1 criteria) at least as high as normal liver uptake assessed within 3 months prior to randomization. Any of the RLI modalities as available (some examples are listed below) can be used as per local practice: * \[68Ga\]Ga-DOTA-TOC PET/CT or PET/MRI * \[68Ga\]Ga-DOTA-TATE PET/CT or PET/MRI * \[64Cu\]Cu-DOTA-TATE PET/CT or PET/MRI * Somatostatin receptor scintigraphy (SRS) (planar and/or SPECT/CT) with \[111In\]In-pentetreotide * SRS (planar and/or SPECT/CT) with \[99mTc\]Tc-octreotide. * Adequate bone marrow and organ function as defined by the following laboratory values prior to receiving the first study treatment: * White blood cell (WBC) count ≥ 2 x 109/L * Platelet count ≥ 75 x 109/L * Hemoglobin (Hb) ≥ 8 g/dL * Creatinine clearance \> 40 mL/min calculated by the Cockcroft Gault method * Total bilirubin ≤ 3 x ULN * Potassium within normal limits. Potassium level of up to 6.0 millimoles per liter (mmol/L) is acceptable at study entry if associated with creatinine clearance within normal limits calculated using Cockcroft-Gault formula. Mild decrease (grade 1) below lower limit of normal (LLN) is acceptable at study entry if considered not clinically significant by Investigator. * ECOG performance status 0-1. * Presence of at least 1 measurable site of disease. Exclusion Criteria: * Prior administration of a therapeutic radiopharmaceutical for GEP-NET at any time prior to randomization in the study. * Any previous therapy with interferons, mTOR-inhibitors, chemotherapy or other systemic therapies except somatostatin analogues (SSAs) of GEP-NET. If as per Investigator's opinion a participant is candidate for such therapies, such participant must not be enrolled. * Participant who received more than 4 cycles of prior SSAs (e.g., octreotide long-acting release) are not eligible. In addition, any participant receiving treatment with short-acting octreotide, which cannot be interrupted for 24 h before the administration of \[177Lu\]Lu-DOTA-TATE, or any participant receiving treatment with SSAs, which cannot be interrupted for at least 4 weeks before the administration of \[177Lu\]Lu-DOTA-TATE. * Documented RECIST v1.1 progression during previous SSA treatments for the current GEP-NET at any time prior to randomization. * Any previous radioembolization, chemoembolization and radiofrequency ablation for GEP-NET. * Any major surgery within 12 weeks prior to randomization in the study. * Known brain metastases. * Participant with known intolerance to CT scans with intravenous (i.v.) contrast due to allergic reaction or renal insufficiency. If such a participant can be imaged with MRI, then the participant would not be excluded. * Hypersensitivity to any somatostatin analogues, to the Investigational Medicinal Products (IMPs) active substance or to any of the excipients. * Active severe urinary incontinence, severe voiding dysfunction, or urinary obstruction requiring an indwelling/condom catheter that, in the judgment of the Investigator, could prevent adhering to radiation safety instructions. Other protocol-defined Inclusion/Exclusion criteria may apply.