This page of ReSharper options allows you to choose the keyboard shortcuts scheme.
Use Alt+R shortcut to open ReSharper main menu
If you use Visual Studio 2019 or later, the ReSharper menu gets hidden inside the Extensions menu, that's why ReSharper allows you to press Alt+R to quickly access its menu commands.
Note that Alt+R is used in some Visual Studio UI elements, for example, in the Find and Replace popup it will replace the next occurrence. If you use it this way or if you have assigned it to another action, disable this option.