no imei can be three things. this is a hardware issue. 1st it can be the modem failing to communicate with the baseband memory chip (eeprom) 2nd it can be the baseband memory chip (eeprom) and 3rd it can be baseband pmu power chip. goto settings / about / and look if you have modem firmware. no firmware usually is the baseband power chip. iTunes error codes will confirm this but will brick the phone in recovery mode. you will need to take it to a specialist for repair if you have had no luck with Apple
=== Update (01/19/2017) ===
Andrew you need to first test the DC-DC convertor Qpoet Voltages to the power amplifiers. low band mid band and high band power supplies come from here as all amps are connected via the Qpoet if one fails they all fail test for shorts in caps around Qpoet then test the voltages to caps during restore via 3utools after 20% as all base band signals will be active at the 20% and above firmware flash. if no shorts are in the amplifier power supplies then go to the antenna switches and test the supplies for short and voltages. i change the switch so i can eliminate this as a cause as it does a lot of switching between 4g/3g high band amp and mid band amp.the midband amp i have found is the culprit of a short in most cases but the switch and amp fail more from use. if you have a scope you can always test the mosi master out slave in data signal and miso master in slave out the master is the controller the slave is the peripheral or utility. the mosi miso lines in the in the rf circuit are the baseband modem being the master as it is the processor and the slave running any other utility. if the processor is transmitting a pairing signal then the modem is good and sending the hello signal. if the same signal is not returning from the slave signal then the chip will not operate. it is a echo signal; so the processor receives the same signal it sent a on switch via a bridge through the ic. next look to the transceiver power supplies and mosi miso data lines this will show you if there is a break in the data line and where the data is vacant the issue is there. then last i would reball the modem if any modem mosi lines are without data. test every fault through voltage to ic and comunication via the miso mosi
no imei can be three things. this is a hardware issue. 1st it can be the modem failing to communicate with the baseband memory chip (eeprom) 2nd it can be the baseband memory chip (eeprom) and 3rd it can be baseband pmu power chip. goto settings / about / and look if you have modem firmware. no firmware usually is the baseband power chip. iTunes error codes will confirm this but will brick the phone in recovery mode. you will need to take it to a specialist for repair if you have had no luck with Apple
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=== Update (01/19/2017) ===
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Andrew you need to first test the DC-DC convertor Qpoet Voltages to the power amplifiers. low band mid band and high band power supplies come from here as all amps are connected via the Qpoet if one fails they all fail test for shorts in caps around Qpoet then test the voltages to caps during restore via 3utools after 20% as all base band signals will be active at the 20% and above firmware flash. if no shorts are in the amplifier power supplies then go to the antenna switches and test the supplies for short and voltages. i change the switch so i can eliminate this as a cause as it does a lot of switching between 4g/3g high band amp and mid band amp.the midband amp i have found is the culprit of a short in most cases but the switch and amp fail more from use. if you have a scope you can always test the mosi master out slave in data signal and miso master in slave out the master is the controller the slave is the peripheral or utility. the mosi miso lines in the in the rf circuit are the baseband modem being the master as it is the processor and the slave running any other utility. if the processor is transmitting a pairing signal then the modem is good and sending the hello signal. if the same signal is not returning from the slave signal then the chip will not operate. it is a echo signal; so the processor receives the same signal it sent a on switch via a bridge through the ic. next look to the transceiver power supplies and mosi miso data lines this will show you if there is a break in the data line and where the data is vacant the issue is there. then last i would reball the modem if any modem mosi lines are without data. test every fault through voltage to ic and comunication via the miso mosi
no imei can be three things. this is a hardware issue. 1st it can be the modem failing to communicate with the baseband memory chip (eeprom) 2nd it can be the baseband memory chip (eeprom) and 3rd it can be baseband pmu power chip. goto settings / about / and look if you have modem firmware. no firmware usually is the baseband power chip. iTunes error codes will confirm this but will brick the phone in recovery mode. you will need to take it to a specialist for repair if you have had no luck with Apple