Yes, iCloud Photos and iTunes Syncing are not compatible. Do not enable iCloud Photos, until you have saved the synced photos somewhere else. iTunes syncing is a one-way syncing. It is easy to get the photos onto the iPhone, but very tedious to get them back to a computer.
Did you keep a backup copy of the synced photos on an external volume? If you still have the original photos on a disk or a stick or somewhere, borrow a computer to upload the originals to your iCloud Photos Library. You can do it using any web browser, where you can open www.icloud.com and use the upload button in Photos. It would be better to upload the original files to iCloud than using the synced files, because Photos may have reduced the size of the synced photos and they may be in a lesser quality.
If you absolutely need to save the synced photos on your iPhone, you have to copy them to the camera roll. Apply a slight edit to the synced photo, and that will add a copy to the camera roll, which will not be removed, when you turn on iCloud Photos. But it will be a lot of work, because it has to be done for each photo individually.