Attack a convoy in 1944 ?? itīs not a madness, itīs a suicide.
and using a IXD boat...
The only way is using long range firing and stay very deep and silent, but is not easy because the allied sonar and asdic is very advanced in this time.
With this large boats the best is go to caribbean, africa or india , where the allied air cover is not very dense and you can find some opportunity target.
if you want convoy battles in the atlantic and have a chance to survive you have to use a VII boat with snorkel and radar detectors.
if you want a bit of "cheat" another possibility is use a XXI in summer 1944.
this is a huge difference and you will have a lot more chances.