You have taken a reasonable course of action and sending your communications by lettre recommandée (recorded delivery) is always advisable.
Without this many companies will ignore correspondence of a contractual nature (ideally you should also pay for the version avec avis de réception - where you receive a slip in the post to tell you when it has been delivered).
However, while Orange has an English-speaking helpline, your letter accompanying the returned modem will probably have gone to a receiving office and, if your letter was not in French, may simply have been discarded.
Similarly, if your letter requesting your service be suspended was in English this may have resulted in it not being acknowledged.
Note also that Orange no longer (as of May 2018) has the option to suspend your broadband service while you are away, if that was your intention.
I suggest you check all correspondence you have received since last spring to see if they formally notified you of this.
If they did then it weakens your argument.
In any event it would be reasonable to expect Orange to have responded to your letter advising that they cannot suspend the service and you have a choice of cancelling or continuing to pay the charges.
The outcome of your calls to the customer service team appears to be unsatisfactory.
Given the fact that you have complaints on several counts, it is appropriate to take the matter forward.
You can complain about poor service at tinyurl.com/y5v3ne94. If you are not satisfied with how your complaint has been dealt with you can progress it to the mediateur-telecom (mediation-telecom.org).
These communications have to be made in French, of course, and you should preferably include copies of any letters you have written and of the receipts of posting.
Question answered by Bob Elliott from the telephone and broadband provider, UK Telecom. See uktelecom.net for more information on their services in France. If you have a query on this topic send it to firstname.lastname@example.org