Is it permissible for a person suffering from a chronic illness, who does not have a stable income or health insurance, to deposit a sum of money in a bank to receive fixed monthly profits to cover treatment and living expenses? And what is the legitimate (Sharia-compliant) solution to secure these expenses if that is not permissible?

It is permissible to deposit money into an investment account with an Islamic bank to benefit from its profits, or to entrust it to a trustworthy person who will invest it, with the profit shared between you according to an agreement. It is forbidden to deposit money in usurious banks for usurious interest or any other reason, because those who deal in usury are at war with Allah the Almighty. Allah the Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of usury, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged." The sustenance of a servant is guaranteed, as the Messenger of Allah, peace and blessings be upon him, said: "Indeed, sustenance seeks the servant more eagerly than his appointed time of death seeks him," and he said: "Indeed, the Holy Spirit inspired in my heart that no soul will die until it completes its term and receives its full sustenance. So fear Allah and be graceful in seeking [sustenance]."