Hi, I have this monstera in my bedroom and it is growing and growing, but the branches/leaves are not being supported by the stem. I have had to tie it up to a radiator (see photos) as it won't stand up on it's own. I haven't pruned it at all and think that is the answer, but also looking for a long term solution for stem to support growth. Any help? Ideas? Thanks in advance!

You must stake it or attach it to something. It is basically a vine and will never support itself. It must grow ON something.

Nice looking plant, beautiful leaves.

You pretty much either either support them with stakes or larger plants or tying them off the way you have, or let them sprawl.

And they'll definitely sprawl.

I love them and have maybe 10 between home and work.

That said they can be a pain to support.

I gave up on my oldest and she sprawls now:)

In your case it's gone way beyond something like a moss pole. Unless you want that attaching directly to your wall, I'd invest in either a large untreated cedar plank that it can attach to, or a large piece of decorative freshwater driftwood. Your plant has an inherent weakness in that it's basically what is called a butt cut. It's a new shoot that formed off an old stump and grew into a new plant. Where it attaches to the old meristem is an area of weakness that may eventually snap if it gets too heavy. It won't kill your plant, your plant has already rooted but it would let it fall over.
One thing that would help besides the large support would be to take all the long adventitious roots going everywhere and burying the ends in the pot. You will also need a bigger pot, you may as well address that when you install the support

There are threads here, and surely videos on the web, about attaching a pole (plank) to a heavy duty nursery type pot. Then set that inside a decorative pot.

Serious monstera GOALS here, holy wow! Nice plant! Like others said, it will never support itself. If you look up images of monsteras in their natural/wild habitats, you'll see they are climbing vines basically. They climb up trees, buildings, anything vertical. Moss poles are common supports for them as houseplants but yours would need a very sturdy rigging, and in my experience, moss poles aren't really that sturdy. I personally dream of attaching supports to my walls and just letting my monstera climb as high as it can but I know that's a little **extra**

If you are handy, you might be able to devise your own rigging for support. If you are not, you could look into some sturdy trellises that can be put indoors.

Kind of like these here around my town