Question

notwithstanding his illness, he attended the meeting.
change into complex sentence​

Answer 1

He attended the meeting, nonwithstanding his illness.

Your sentence is already a complex sentence!

If you mean to say change to a compound sentence, you may say:-

He attended the meeting although he had an illness

Use FANBOYS TEA

FOR

AND

NOR

BUT

OR

YET

SO

Though

Erstwhile

Although