Boxes 23 onwards only need to be completed if you are UK resident and claiming the remittance basis.
If this is not the case then you do not need to enter any information in these sections.
Guidances notes on this can be found here
Residence, remittance basis etc (Self Assessment SA109)
You would still complete boxes 18/19, boxes 20-22 are only for income which potentially will be doubly taxed.
These pages as suggested can only be completed online through third party software as HMRC software does not have this section.
You can complete a paper copy of the return instead or purchase third party software
Self Assessment commercial software suppliers
Deadlines for self assement filing can be found here
Self Assessment tax returns