Düsseldorf - Haldern is by free Regional Express.
Journey time ~ 1 hour.
There are direct trains or trains with one change.
For details use the schedule planer: https://en.rail.cc/interrail-train-connections
Paris-Düsseldorf is possible by expensive THALYS train. Cheaper solutions as well via the following link.
First travel Paris-Cologne, there change to a free train to Düsseldorf (about 30 minutes by free Regiona Express):paris-cologne-koeln-train-t3993.html
A good connection directly from Paris to Haldern without supplements
is this one:
Paris Est Dep: 0735 | free Regional Express
Bar-le-Duc Arr: 0951
Bar-le-Duc Dep: 1010 | free Regional Express
Nancy Ville Arr: 1117
Nancy Ville Dep: 1220 | free Regional Express
Luxembourg Arr: 1345
Luxembourg Dep: 1424 | free IC train
Oberhausen Hbf Arr: 1835
Oberhausen Hbf Dep: 1851 | free Regional Bahn
Haldern(Rheinl) Arr: 1933 Book your official online-ticket
for this route directly in the railcc-Bahn shop (including special price tickets and specials like "Europe Special"): https://bahn.rail.cc The price for you is the same like at DB. But you directly support the railcc project with your purchase. Thank you!