Right from the beginning there have been rules for solo playing D&D. These rules are intentionally faster and lighter than others available in the DMs Guild.
These are not the first solo rules for Dungeons and Dragons. There have been solo play rules build into the game right from the first edition.
For a genuine DM free role-playing experience though you need some additional assistance, more than just random rooms and corridors.
These are not the only solo rules available. The granddaddy of them all is the Mythic Game Master Emulator (GME). All solo role laying games owe some small debt to Mythic’s GME. An alternative is 5E Solo Gamebooks The Solo Adventurer’s Toolbox. That gives you nearly 170 pages of rules and tables of D&D specific solo role play tools.
These rules are different but if you have the Adventurers Toolbox and you like it then you do not need this book.
The thing that makes this set of solo rules different is that I find too many tables and too many dice rolls break the suspension of disbelief. For me the story is more important the scrolling through hundreds of pages for the right table.