Try not setting an APN, that should use the default APN stored in the SIM card.
Also test the SIM in another device in the same location to be sure that the network works. Confirm the band being used locally is supported and enabled in the modem.
FWIW
Last time I traveled I could get IPv4 but, unlike my phone, not IPv6.
I travel again in June and will try again both IPv4 and IPv6.
(IPv4 and IPv6 work for my provider my country)
Or, if, by any chance, you can have access to a wired LAN with internet connection elsewhere, then, you can use and configure the GL-X750v2 WAN ethernet port to connect to that LAN and access the Internet.
This is how I do it when upgrading my (older) GL-X750 because some 4G specific driver needs to be downloaded, and, that temporarily poses similar chicken and egg problem: no internet connection to download the required packages
For a box with mobile radio that looks reasonable indeed :-). In the notes I made for first install (few years ago) uqmi was not part of the image. Perhaps today it is. In any case, it was certainly a suggested package for such 4G/LTE box and OP mentions the device works in Germany. Then chances are big @Surfer2010 installed it already indeed ...
lsusb
Bus 001 Device 004: ID 2c7c:0306 Quectel EP06-E
Bus 001 Device 003: ID 05e3:0752 Generic USB Storage
Bus 001 Device 002: ID 05e3:0618 USB2.0 Hub
Bus 001 Device 001: ID 1d6b:0002 Linux 5.10.104 ehci_hcd EHCI Host Controller
root@OpenWrt:~# dmesg | grep tty
[ 0.000000] Kernel command line: console=ttyS0,115200n8 rootfstype=squashfs,jffs2
[ 0.348655] printk: console [ttyS0] disabled
[ 0.353299] 18020000.uart: ttyS0 at MMIO 0x18020000 (irq = 9, base_baud = 1562500) is a 16550A
[ 0.362402] printk: console [ttyS0] enabled
root@OpenWrt:~# picocom -b 115200 -r -l /dev/ttyS0
root@OpenWrt:~# picocom -b 115200 -r -l /dev/ttyS0
picocom v3.1
port is : /dev/ttyS0
flowcontrol : none
baudrate is : 115200
parity is : none
databits are : 8
stopbits are : 1
escape is : C-a
local echo is : no
noinit is : no
noreset is : yes
hangup is : no
nolock is : yes
send_cmd is : sz -vv
receive_cmd is : rz -vv -E
imap is :
omap is :
emap is : crcrlf,delbs,
logfile is : none
initstring : none
exit_after is : not set
exit is : no
Type [C-a] [C-h] to see available commands
Terminal ready
Seems roaming to COSMOTE 202 1 is true ... not_forbidden looks good, not sure what not_preferred means.
If you put the SIM in a smartphone. Does it work? And, if so, using which provider?
Also COSMOTE?
If your phone then searches for any network APN. Maybe another one (also) works?
I experienced years ago European roaming went flawless, but it seems, meanwhile, more and more providers have silent deals to prefer or forbid other ones abroad, and sometimes they only offer restricted service (say 4G not 4G+ or IPv4 not IPv6). I suppose this is not legal but it is.
Whichever providers have strong signal where you are, hopefully allow you to roam.