Depends if you’re on ILP or Standalone policy. Normally 1 mth after the last due date before it lapses
If you’ve been diagnosed with cancer before, chances for you to apply for any insurance is very slim / NIL.

the MRTA will not be approved. You may then proceed with the loan without MRTA

It's almost not possible to get any life/ 45 Critical Illness coverage from any company for 20 years duration of diagnosis. However, if your existing policy is under Great Eastern, you may get special offering of policy that is without health assessment, covering life and 45 Critical Illness(with exclusion of pre-existing illnesses). Example if Great Eastern is offering you a plan that is covering Life and 45 Critical Illness, you wouldn't be able to claim for the "cancer" but can claim for the rest of the 44 types of Critical Illness, well, it's fair enough I would say.

This kind of policy will open to public purchase but those selected customer by Great Eastern will have no health assessment. Some of my cancer client bought quite a lot during the offering. So, if you are Great Eastern policy holder, do keep in touch with your agent and remind your agent to update you.

Some bank officer do manage to apply MRTA for you, but it's without declaring your current health condition, this is kind of common but unethical practice back when I was a mortgage agent.