As opposed to the Resident Evil 2 remake, Devil May Cry 5 did not launch with ultrawide support, even though it uses Capcom's proprietary RE engine.
This is bound to have let at least some owners of ultrawide monitors down, however, it does seem that a player-made ultrawide fix has emerged in the meantime.
Does Devil May Cry 5 have Ultrawide Support?
An ultrawide support fix for Devil May Cry 5 has appeared on Steam, while players wait for Capcom to potentially add an official solution to their resolution woes, and it turns out it is quite easy to use.
Open it with your software of choice (or 7Zip if you have it), and you'll find a series of executables titled after several ultrawide resolutions. Simply paste the executable of the resolution you want to use in the game's folder. By default, it should look something like Drive:\Program Files\Steam\steamapps\common\Devil May Cry 5\.
After copying the executable, simply launch the game using it and change the resolution to your desired one while in-game. As it's a player-made ultrawide fix, and not an official patch for Devil May Cry 5, you may run into issuess. At the moment, however, it seems to be the only options for playing Devil May Cry 5 using ultrawide resolutions.