I usually use the drainage stuff instead, they do more variety of bends such as 15 & 30 degrees. Can be useful when a 45 is too much. As it's not on show it doesn't matter.
https://www.screwfix.com/c/heating-...category=cat831514&page_size=20&page_start=20
As others have said, the adaptor that comes with the frame will be push fit so ensure it goes into a pushfit socket and not glued.
You may be as well removing the white pipe from the soil junction and renewing from there with new 4 inch and the required offsets.