Type 2 Diabetes Mellitus (T2DM) is a major chronic lifestyle-related disorder with a significant impact on quality and costs of care. As patients with T2DM often have insufficient knowledge about proper self-management and are insufficiently motivated for a lifestyle change, interventions with more motivational strategies and personalization are needed. The use of real-time monitoring of glucose values, nutrition and physical activity in combination with coaching aimed at lifestyle-related behavior change may improve patients' diabetes management. Therefore, ZGT, UT and RRD developed the Diameter app. The aim of phase 2 of this study is to investigate the Diameter as blended-care using a feasibility study. The primary objective of this feasibility study is to assess intervention usage and acceptability of the Diameter as a blended-care intervention in secondary care, of which some are also following a combined lifestyle intervention (GLI). Secondary objectives are to explore behavioral (e.g. physical activity), physiological (e.g. BMI), psychological (e.g. health-related quality of life) and clinical outcomes (e.g. glucose control, estimated HbA1c values). This study has a mixed-method design with 3 (regular participants) or 4 (participants who decide to follow the Combined Lifestyle Intervention (GLI) COOL next to the Diameter during the study period) data collection points. Patients will start with a two-week period of baseline measurements. Subsequently, patients will use the Diameter as a blended-care intervention for 10 weeks. The two-week measurement periods will be repeated twice (T1: week 13-14 and at T2: week 25-26). Between T1 and T2, patients will use a version of the Diameter without daily coaching messages. At T1 and T2, questionnaires will be administered, data on physical activity, food intake and glucose values will be logged, and blood and urine samples will be retrieved from regular care measurements. In addition, open-ended interviews will be performed with 10-15 patients at T1. For participants who also decided to follow the COOL program, some routinely collected measurements as part of the COOL program will be obtained from the patient record.