US dad-volunteer in Africa-child sup adv

My ex wife remarried and their needs are comfortably met. When we div the agreement was that 1 pay 1k/mo for our 2 kids until the one turns 18 (just happened).Payments were to begin in Nov 06, but I have no income here. Ive been helping in Africa for 4+ years. I asked my ex to include cash I paid prev to help her through a tough time and apply it to cover hese first few months. then I asked if I could reduce the amount on my remaining child under 18 to something like 200/month- paid irregularly as I can find something from time totime. she knows I have nothing yet is acting like she is shocked.
I looked on the calculator, and even if I was earning, the amount would be around 200/250 at most
-I couldn't afford to come last year, but in the 2 prev years I had to spend a fortune- about 7k- to have them out in Calif for 6 weeks with me- hotel, food, gas, 1800 RT air from Africa, etc. I understand there should be some allowance for that- is that true. the jurisdiction is Ohio. Which agency/contact should I talk to /email to try to reach an agreement to modify the arrangement. Since it wasnt a court-ordered enforcement, just an agreement between us, can I xtill petition for the amounts to be modified as to the reality of the situation?
Thanks for your thoughts.

Yes you can ask her to reduce it if she does not agree you may go to court and get an amount set based on your income and prospects etc. --best to get it settled if possible so she does not go to court and seek back pay etc.