Here are some helpful suggestions:
1. ADF will often rest on things. ADF also come from areas that often have exposed roots and wood in the water. Consider getting some wood like manzinta that will help to make resting and hiding places for them.
2. Attaching plants to the driftwood mentioned in point 1. will also help provide them places to lay. It is not uncommon for me to find my ADF resting on the leaves or by the roots of my anubias.
3. You may also use rock to help provide different levels of access for your african dwarf frog. Many will welcome smooth rocks, however I have had no issue ever with rocks such as dragonstone.
4. African dwarf frog in my experience seem really to benefit from floating plants. They will often rest themselves in the roots of the floating plants which will bring them closer to the surface. Also sometimes they may even prop themselves up slightly on the edges of the floating plants. I particularly like frogbit and dwarf water lettuce for african dwarf frogs.
If I was going to do a planted tank with African Dwarf Frogs, I would consider doing low light plants such as crypts. Based on my own personal experience, you see significant behavior out of ADF in subdued lighting. I also believe that you will see significant more activity out of African Dwarf Frogs when your lights have a morning/evening ramping period. Based on this, if I was to recommend a light for a 20 gallon, my go to light recommendation would be the Fluval Aquasky since that light is easy to program, has a ramping period, its light distribution is great, and seems to promote excellent growth in lowlight plants.
If you do decide to go the route of aquasky for your tank and do low light plants like crypts, then check out Bentley Pascoe's channel. He has a fantastic guide on the Fluval Aquasky series.