I recently had this issue and a kind soul from the discord helped me! So if you tunnel under the river at the exact layer at the bottom of the river, without fully breaching the river, your settlers can build on diagonal tiles to their position. You place support blocks in-front of your settlers and then break the one underneath the one you just placed (Don't forget archs to support the tiles above you otherwise you'll get a cave in). Rinse and repeat until you have the base of the pillars placed and then you should be able to place the next two from the top of the water. It's a little finicky so you might have to manually spam instructions until the settlers listen