FAA rule for radio communications failure

Two-way radio communications failure[edit]

Section 91.185 of the Federal Aviation Restrictions deals with loss of
radio communications while in flight. If a loss of radio
communications were to be encountered during VFR conditions, or if VFR
conditions are encountered after loss of communication with the ground
and other aircraft, the pilot of the aircraft shall continue the
flight under VFR and land as soon as practicable. If, however, the
failure occurs in IFR conditions and/or the VFR conditions are not
forthcoming, the pilot should continue under the following conditions:

Route – The pilot will follow:

The route assigned in the last contact with ATC before loss of
communication, or, if being radar vectored, continue direct to the
radar fix specified in the vector clearance;
In the absence of an assigned route, the pilot will follow the route
advised by ATC;
In the absence of an ATC assigned or advised route, the pilot will
follow the route set down in the flight plan.

Altitude – The pilot will continue at the highest of the following
altitudes or flight levels:

The altitude assigned in the last contact with ATC before loss of communication;
The minimum altitude for IFR operations;
The altitude advised by ATC to be expected in a further clearance.[17]

from https://en.wikipedia.org/wiki/Federal_Aviation_Regulations