Sorry @crispyoz, maybe I was not clear, my unit does not display this row, this is all it shows:

  ____       _             ____
 / __ \___  (_)__  ___    / __ \__ _  ___ ___ ____ _
/ /_/ / _ \/ / _ \/ _ \  / /_/ /  ' \/ -_) _ `/ _ `/
\____/_//_/_/\___/_//_/  \____/_/_/_/\__/\_, /\_,_/
W H A T  W I L L  Y O U  I N V E N T ? /___/"

Board: Onion Omega2 APSoC DRAM:  128 MB
relocate_code Pointer at: 87f5c000
flash manufacture id: c2, device id 20 19
find flash: MX25L25635E
*** Warning - bad CRC, using default environment

============================================ 
Onion Omega2 UBoot Version: 4.3.0.3
-------------------------------------------- 
ASIC 7628_MP (Port5<->None)
DRAM component: 1024 Mbits DDR, width 16
DRAM bus: 16 bit
Total memory: 128 MBytes
Flash component: SPI Flash
Date:Sep 19 2017  Time:15:11:41
============================================ 
icache: sets:512, ways:4, linesz:32 ,total:65536
dcache: sets:256, ways:4, linesz:32 ,total:32768 
CPU freq = 575 MHZ
Estimated memory size = 128 Mbytes
Resetting MT7628 PHY.
Initializing MT7688 GPIO system.
wifi mac address = 004C02084D10.


**************************************
* Hold Reset button for more options *
*************************************

Boot Linux from Flash NO RESET PRESSED.
## Booting image at bc050000 ...
Bad Magic Number,0400AC01

And nothing else.

I tried to enter safe mode from putty in windows and from screen in Ubuntu and in both cases it shows:

Please select option:
   [ Enter ]: Boot Omega2.
   [ 0 ]: Start Web recovery mode.
   [ 1 ]: Start command line mode.
   [ 2 ]: Flash firmware from USB storage.

Option [f] selected.

Boot Linux from Flash.
## Booting image at bc050000 ...
Bad Magic Number,0400AC00

And it doesn't work.
Are there any ways to copy the firmware from the USB stick to the linux partition of the Omega2 through the u-boot command line?

@crispyoz sorry if I answer you only now and thank you for your help (thank you also @jmztaylor), anyway I tried an infinite number of times to press F and [Enter] to access the failsafe mode but it doesen't work. Looking at the serial capture that I uploaded, you can also notice how the booting process doesn't even initialize the Kernel. For this reason it does not display the line (I have found it on the internet) :

Press the [f] key and hit [enter] to enter failsafe mode                        
Press the [1], [2], [3] or [4] key and hit [enter] to select the debug[    5.58)
 level

It definitly stop here and than it doesn't show anything else:

Boot Linux from Flash NO RESET PRESSED.
## Booting image at bc050000 ...
Bad Magic Number,0400AC01

@christian-lacdael Have you ever resolved this issue?

I have never used this particular Onion Omega2+ but when I turn it on for the first time I noticed that I wasn't able to find the wireless network generated by the Omega.
When I connect it to serial (I'm using the expansion dock) it shows:

  ____       _             ____
  / __ \___  (_)__  ___    / __ \__ _  ___ ___ ____ _
 / /_/ / _ \/ / _ \/ _ \  / /_/ /  ' \/ -_) _ `/ _ `/
 \____/_//_/_/\___/_//_/  \____/_/_/_/\__/\_, /\_,_/
 W H A T  W I L L  Y O U  I N V E N T ? /___/"

Board: Onion Omega2 APSoC DRAM:  128 MB
relocate_code Pointer at: 87f5c000
flash manufacture id: c2, device id 20 19
find flash: MX25L25635E
*** Warning - bad CRC, using default environment

============================================ 
Onion Omega2 UBoot Version: 4.3.0.3
-------------------------------------------- 
ASIC 7628_MP (Port5<->None)
DRAM component: 1024 Mbits DDR, width 16
DRAM bus: 16 bit
Total memory: 128 MBytes
Flash component: SPI Flash
Date:Sep 19 2017  Time:15:11:41
============================================ 
icache: sets:512, ways:4, linesz:32 ,total:65536
dcache: sets:256, ways:4, linesz:32 ,total:32768 
CPU freq = 575 MHZ
Estimated memory size = 128 Mbytes
Resetting MT7628 PHY.
Initializing MT7688 GPIO system.
wifi mac address = 004C02084D10.


**************************************
* Hold Reset button for more options *
*************************************

Boot Linux from Flash NO RESET PRESSED.
## Booting image at bc050000 ...
Bad Magic Number,0400AC01

So I tried to flash the last firmware from USB, as showed in the docs, but when I do it the Omega give me this error:

***************************************
* [!] This will take several minutes  *
* please do not power off your Omega2 *
***************************************


*ERROR: CTL:TIMEOUT X
BBB_reset
usb_control_msg: request: 0xFF, requesttype: 0x21
value 0x0 index 0x0 length 0x0
ERROR: CTL:TIMEOUT X

I have tried with 3 different types of USB stick and it always shows the same error.
I am working to a university project and, due to the lockdown, is really difficult to receive more material from the campus so I don't have the ethernet shield to access to the Web recovery mode. There is something that I can do to try flashing the bootloader in some other way?
Is it possible to do something from the boot command line? Because I am not even able to access to failsafe mode.

Hi, I've never used the PPP protocol but, if you want to know more about interfacing SIM800/900 with onion omega 2, maybe my project can help you:
https://github.com/AlbertoZandara/Onion-GSM-Module

I realized a GSM module with Omega 2+, maybe this can help you:
https://github.com/AlbertoZandara/Onion-GSM-Module