Is it permissible to inform others about one's charity to help in fundraising, despite the fear of losing the reward for secret charity and of showing off (riya')?

Performing beneficial work to help those in need carries immense reward from Allah. One should persevere in it and continue it. It does not harm if people become aware of it, as long as you do not intend by it ostentation or boasting, but rather closeness to Allah. You will be rewarded for encouraging people and alerting them to the importance of donating, because by doing so, you become a caller to an act of obedience, and you will receive its reward and the reward of those who act upon it. Indeed, displaying charity may be better in some circumstances. And you must beware of Satan's discouragement.