IELTS or Medium of Instruction letter?

Short answer: Some universities admit students with an MOI letter. The embassy in Dhaka is stricter. For the visa it accepts only IELTS, with a score of at least 6, issued within two years before your visa application, unless your university has already checked your English and recorded it on Universitaly (Dhaka checklist).

• The national rule says the embassy may only use certificates showing at least B2 in all four skills, from internationally recognised bodies or from universities (MUR circular). An MOI letter does not test four skills.
• Many master's courses ask for IELTS 6.5. Aim for the course's score, never below 6. My free IELTS course is built for exactly this.