Small Aircraft; Low Wing; 1 Eng; Retractable Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 2.5
Communication Breakdown; Situational Awareness; Time Pressure
ATC Issue All Types; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
Airspace Structure; Procedure; Equipment / Tooling; Human Factors
I took airspace from previous controller. Aircraft X wanted to stay low to limit oxygen and couldn't take longer routing to save on gas. Previous controller let Aircraft X stay at the MIA (Minimum IFR Altitude) per request. Previous controller told Aircraft X he was going to need to climb to avoid terrain; which he acknowledged. In the briefing I was told about an Aircraft X needing to climb prior to terrain. As soon as I took the airspace; I asked Aircraft X if he could take a reroute to avoid terrain. Aircraft X said Center transmissions were starting to breakup. I tried multiple times to relay to Aircraft X to get Aircraft X to either change to a frequency he could hear me on or climb to 11;000 feet. Aircraft X unfortunately was unreachable through relays or ground transmitters. The transmitters were unable to reach Aircraft X at 10;000 feet prior despite MIAs being 10;000 feet. Aircraft X eventually climbed on his own to 11;000 feet after flying through an 11;000 foot MIA for several miles. I was able to relay through another aircraft and get Aircraft X to change to a frequency I could talk to him on. Frequency coverage of one frequency at lower altitudes in this sector is poor when at MIAs. Unless more transmitter sights are put in; the coverage at low altitude will continue to be spotty. Changing aircraft's frequency at low altitudes over sooner would help due to another frequency having better coverage in some spots between sectors.
A ZDV Center Controller reported they lost communication and could not issue a climb clearance to an aircraft due to poor radio coverage in mountainous terrain and the aircraft flew below the Minimum IFR Altitude before climbing on its own.