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Ornamental grasses, including pampas often die out in the center. To correct this, the entire plant is dug up, the dead parts removed, and pieces from the edges are replanted. This is best done while the grass is dormant or in early spring. Either cut the plant down or use twine to tie it up and out of the way. If you've had this for over 5 years, be advised that this is a big task requiring hard labor. It is difficult to separate the dead center; I've used an axe since regular garden tools did not penetrate the very dense root mass. If this is a massive plant, in place over 10 years, suggesting the use of a small backhoe is not overstating the difficulty.

Soil fertility and water management are the keys to plant health and normal growth.
Fertilize and irrigate appropriately and you may see a big difference.

While the plant is dormant for winter, cut it down as low as you can. This can be done in late fall or very early spring. If it is a large plant, and they can get very large, electric hedge clippers or a small chainsaw come in handy. Use twine or a bungee cord to tie up the top. This gives you easier access to the plant base and quicker clean-up.

I prefer to prune just before the new growth comes in for the year. Here is an article that will help you further: https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/pruning-pampas-grass.htm

Pampas grass can be grown in container, however it is always easier to start with divisions from an already established plant. This will get the results you are looking for, faster. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm

You need to select containers that are large enough and have enough volume of good-quality container soil mix to support these grasses. And then be sure to select plant materials that are a zone or two hardier than the zone you live in, as being above ground makes them 1 to 2 zones less hardy.
Good drainage is also necessary as is making sure they get adequate water up until the soil freezes in the planter. The materials that your planter is made up of has a direct connection to the weatherization of it. Composite plastics, metal, and concrete are the most durable, long lasting, and can resist temperature fluctuations. You then line the inside of the container with a 3″ thick Styrofoam.
Mulching the containers in the fall will also help insulate the plant materials and provide moisture retention. And if possible, for winter, group the containers together in a sheltered spot or even in a garage or shed.
You can even wrap them all in bubble wrap to provide additional insulation.