I am uncertain from your posting's wording whether you have a Fiat Ducato-based panel-van conversion (campervan) or a coachbuilt motorhome based on a Fiat Ducato.
Remis markets several REMIcare insect protection door screens and details are here
https://www.remis.de/mobil/en/products/insektenschutztueren
Contact details are provided, so you could try asking Remis if they can give you repair instructions.
Alternatively, you could try contacting LeisureShopDirect, as they market REMIcare products in the UK (example here)
https://www.leisureshopdirect.com/v.../remicare-ii-door-flyscreen-1800-x-650-x-58mm
GOOGLE-searching using
remis door flyscreen fishing line as the search-term will retrieve several on-line discussions that may be useful.