When an aircraft is flying over international waters (high seas) it should follow the ICAO Annex 2 rules of the air. On the other hand, when an aircraft is flying over the territory of a State, it should follow the rules of the air of that State unless they are similar to the ICAO Annex 2 rules. The rules published by the country having jurisdiction over the terrotiry overflown have priority over the rules published in ICAO Annex 2.

This question is similar to 101298:

• communication failure occurs over US territory → follow US rules of the air
• communication failure occurs over international waters (high seas) → follow ICAO Annex 2 rules of the air

ICAO Annex 2
2.1 Territorial application of the rules of the air

2.1.1 The rules of the air shall apply to aircraft bearing the nationality and registration marks of a Contracting State, wherever they may be, to the extent that they do not conflict with the rules published by the State having jurisdiction over the territory overflown.

COMMISSION IMPLEMENTING REGULATION (EU) No 923/2012 (SERA)
Article 1 - Subject matter and scope

2. This Regulation shall apply in particular to airspace users and aircraft engaged in general air traffic:

(a) operating into, within or out of the Union;

(b) bearing the nationality and registration marks of a Member State of the Union, and operating in any airspace to the extent that they do not conflict with the rules published by the country having jurisdiction over the territory overflown.