Why walk? You’ve got a whole set of strong, flexible appendages. What you need to do is
Consider a snake. It has no limbs, and no issue with locomotion. Your octopoid can use its tentacles in much the same way, flexing a few strong, thick locomotion tentacles and pushing along the floor with them. This helps it stay low and camouflaged in a grassland environment.
Or consider a caterpillar. Their undulating motion would work superbly for your octopoid, who could even take advantage of their omnidirectional tentacles to let them change direction near instantly.
Or consider a worm. Reaching out with the fore tentacles, pushing with the rear ones and dragging oneself along the ground might not be the most elegant form of locomotion, but in the rainy season it could be marvellously efficient.
Finally consider the octopus. Many of the ways octop(uses/i/oids/us) move underwater are still usable on land, especially when they snake their tentacles under each other to create the impression of a rolling, roiling Mass of Tentacular Doom.
Your octopoid will probably also be pretty smart, so switching modes or temporarily hoisting itself into the air atop a column of tentacles to temporarily get better vision probably won’t be too much of a challenge for them.