Center Controller reported they lost communications with a C172 and the aircraft descended below its assigned altitude and the minimum IFR altitude.

Narrative

Aircraft X; filed IFR from ZZZ to ZZZ1. They received their clearance and release for departure. As they climbed out from ZZZ; I cleared them direct to the ZZZ VOR. As they climbed to their final altitude; I offered them direct to their destination airport or to leave them over ZZZ [VOR] then direct their destination and advised them that leaving them over ZZZ [VOR] would allow them to fly over lower terrain for a longer period of time but they were above my MIAs (Minimum IFR Altitude) the whole way. The pilot opted for direct to their destination; so I cleared them direct to ZZZ1 [Airport]. Their flight continued as normal and I began the handoff to TRACON. TRACON took the handoff and I attempted to ship Aircraft X at XA44Z. I received no response so I tried a different transmitter and still received no response. I observed Aircraft X at an altitude of 15700 ft. and their filed/assigned altitude was 16000 ft. The 300 foot difference didn't overly concern me due to the fact that they were still on an ZZZ [Airport] altimeter. At XA46Z; I called TRACON to see if Aircraft X had switched to their frequency and hadn't acknowledged my frequency change for them. I was told they were not there yet.I asked Aircraft Y; who was on my frequency to try an airborne relay to Aircraft X and have them contact TRACON. Aircraft Y did not reach Aircraft X. At XA47Z; I observed Aircraft X descend to 15;600 ft. At XA48Z; I observed Aircraft X descend to 15;500 ft. and immediately issued a low altitude alert. At XA50Z I observed Aircraft X descend to 15;400 ft. and issued another low altitude alert. At this time; Aircraft X was still in an MIA area of 16;000. I also had Aircraft Z on frequency; and not knowing where Aircraft X was; asked Aircraft Z to go to the departure frequency that Aircraft X was on when they departed ZZZ; wondering if the pilot had accidentally switched back to that frequency. I asked Aircraft Z to go to the departure frequency and broadcast for Aircraft X to climb immediately to 16;000 ft. Aircraft Z made the broadcast twice on the departure frequency and never got a response. I issued another low altitude alert at XA52Z. At XA56Z I observed Aircraft X at 15;200 ft. and issued another low altitude alert as they continued to descend to 15;100 ft. The aircraft continued to descend to 14;700 ft. still located in the 16;000 ft. MIA on the edge of a 14;000 ft. MIA. As they transitioned into the 14;000 ft. MIA I saw them go as low as 14;100 ft and broadcast 'Aircraft X; maintain altitude 14;000 ft or above; you're in an area where the minimum IFR altitude is 14000 ft' and had no response. At XA58Z I issued another low altitude alert. Several calls were made between myself and TRACON to check comm status as well as inform TRACON when I did finally receive a transmission from Aircraft X. Aircraft X finally said they were with me and they had comm issues. I stated 'Aircraft X; I gave you several low altitude alerts. Are you ok?' He responded that he was ok; so I transferred communications to TRACON due to the fact that my frequency quality at his location and altitude was not very good.