Step 1. Leave Obsidian out of it (out of your syncing problem). Using whatever software you prefer, and whatever server you prefer, sync the folder on your PC with a folder on your phone.
Step 2. Set the folder on your phone as a vault on your phone.
The reason you’ve been advised against syncing a vault from one location into a folder in another location is that the directory structure of one vault is different than the directory structure in the other (because on PC the folder is further down the directory heirarchy). So, the links in various documents may get broken, as you/Obsidian interact with files inside the folder while on one device or the other.
But that’s just an inherent danger of having a vault inside a vault. And having a vault inside a vault is what you want to do–there’s no way around that. So, you will need to just live with that possibility of instability. Depending on what links you have in your folder, maybe you won’t be able to live with that. Or maybe you will–it just depends!
There’s a post on this forum about having vaults inside vaults. Nested vaults - usage and risks - Share & showcase - Obsidian Forum It’s not officially recommended to nest vaults, but the thread itself goes into great detail about the actual potential problems and how to avoid them. I recall link style (wiki-style vs Markdown style) is a part of it, and that option can be set in Obsidian settings.
Depending on what your links inside your folder do, it may not actually be a problem for you.
I have a vault inside a vault on my PC, and it’s no problem at all for me, because the folder/vault inside my vault is for a specific self-contained project–nothing links to anything outside of the folder, nor vice versa. Linking to documents outside the folder is possible, but I have no need to do that, so no need to know how to do it either. And I don’t edit that folder while using the larger vault either.