Sub-menu too deep to display fully on cellular and cannot scroll down it....

Folks:

Goto wwww.AuthorRobertHolland.com with your cell phone browser and tap the < Illustration & Excerpts > menu choice to see the sub-menu, i.e. dropdown menu.

There are 17 entries on that sub-menu and more to come as we create more art for posters. Each one goes to a discreet webpage.

I have tried and tried to create a side-by-side menu for this with no luck, but is that the answer?

Also, I do not understand why this sub-menu will not allow scroll down on a cell phone to see all of the choices.

I have studied this for a coupe of week on and off, but I'm lost. Can you help me out, or perhaps suggest a menu management plugin that works well with GeneratePress Pro to make this submenu functional?

Thanks, as always.