Weird lte connection TP-Link tl-mr6400 v5.3

Hi there, I have recently installed an OpenWrt 23.05.5 customized image (from firmware-selector.openwrt.org) with the default packages + luci-proto-qmi and qmi-utils, which are needed to get the lte module to work (and maybe should be added to the default image for this router?), then added a new interface with these settings in /etc/config/network:

config interface 'wwan'
	option proto 'qmi'
	option device '/dev/cdc-wdm0'
	option apn 'internet.it'
	option auth 'none'
	option pdptype 'ipv4v6'
	option dhcp '0'

and this interface works but... well... weirdly. What I mean by that is this: the router mostly has internet access (as do all connected devices), but not to every website, and not always. Let's take for example www.speedtest.net: on the router it does correctly resolve to an ipv4 address and ping works fine, on a connected laptop it also resolves to the same ipv4 address but ping does not work, as does curl, as does going to it in a browser. Furthermore, Firefox shows a strange hourglass icon which I've never seen before:
20241209_23h46m51s_grim
with a blank page. This doesn't happen on youtube.com, openwrt.org, google.com, and a bunch of other sites; they work just fine, which tells me the router does in fact have internet connectivity, just, you know, a weird one.
The only interesting things from the logs when restarting the interface seems to be these:

Mon Dec  9 23:54:00 2024 daemon.notice netifd: wwan (18877): Stopping network wwan
*** Mon Dec  9 23:54:01 2024 daemon.notice netifd: wwan (18877): Command failed: ubus call network.interface notify_proto { "action": 0, "link-up": false, "keep": false, "interface": "wwan" } (Permission denied) ***
Mon Dec  9 23:54:01 2024 daemon.notice netifd: Interface 'wwan' is now down
Mon Dec  9 23:54:01 2024 daemon.notice netifd: Interface 'wwan' is setting up now
Mon Dec  9 23:54:01 2024 daemon.notice netifd: wwan (18927): Waiting for SIM initialization
*** Mon Dec  9 23:54:02 2024 daemon.notice netifd: wwan (18927): Failed to parse message data ***
Mon Dec  9 23:54:02 2024 daemon.notice netifd: wwan (18927): PIN verification is disabled
*** Mon Dec  9 23:54:02 2024 daemon.notice netifd: wwan (18927): Device does not support 802.3 mode. Informing driver of raw-ip only for wwan0 .. ***
Mon Dec  9 23:54:03 2024 daemon.notice netifd: wwan (18927): Waiting for network registration
Mon Dec  9 23:54:04 2024 daemon.notice netifd: wwan (18927): Starting network wwan
Mon Dec  9 23:54:04 2024 daemon.notice netifd: wwan (18927): Setting up wwan0
Mon Dec  9 23:54:04 2024 daemon.notice netifd: Interface 'wwan' is now up

but they don't seem like a severe warning sign to me, and can just as well be standard operation. uqmi --device /dev/cdc-wdm0 --get-data-status shows "connected", and uqmi --device /dev/cdc-wdm0 --get-signal-info shows

{
	"type": "lte",
	"rssi": -71,
	"rsrq": -11,
	"rsrp": -103,
	"snr": 4.800000
}

all pretty standard, nothing to see here.
I've read from other posts on this forum (e.g. Establishing LTE connection with TL-MR6400 v5 LTE, [TL-MR6400 v5] LTE “disconnect” after six hours) that someone suggests to install the mrhaav's uqmi package (which btw is currenlty available for up to 23.05.04), and I did, but the problem persists.
So, has anyone had this issue with this particular model? I did find similar topics but none with this exact problem (because the lte module is in fact working). Maybe I should also point out that the router worked fine with the stock firmware as does the sim card in a cell phone, and I never have had this problem before. Thanks in advance to who can solve this mistery :upside_down_face:, and let me know if you need additional info.

Update:
I reflashed the same image (tftp recovery, not sysupgrade) just to try something, and now I get this in the logs:

daemon.notice netifd: wwan (2905): Stopping network wwan
*** daemon.notice netifd: wwan (2905): Command failed: ubus call network.interface notify_proto { "action": 0, "link-up": false, "keep": false, "interface": "wwan" } (Permission denied) ***
daemon.notice netifd: Interface 'wwan' is now down
daemon.notice netifd: Interface 'wwan' is setting up now
daemon.notice netifd: wwan (2955): Waiting for SIM initialization
*** daemon.notice netifd: wwan (2955): Failed to parse message data ***
daemon.notice netifd: wwan (2955): PIN verification is disabled
daemon.notice netifd: wwan (2955): Device does not support 802.3 mode. Informing driver of raw-ip only for wwan0 ..
*** daemon.notice netifd: wwan (2955): sh: write error: Resource busy ***
*** kern.err kernel: [  432.210368] qmi_wwan 1-1:1.4 wwan0: Cannot change a running device ***
daemon.notice netifd: wwan (2955): Waiting for network registration
daemon.notice netifd: wwan (2955): Starting network wwan
daemon.notice netifd: wwan (2955): Setting up wwan0
daemon.notice netifd: Interface 'wwan' is now up

A kernel error! Serious buisness...
And now even the router doesn't ping nor resolve anything. Again uqmi shows everything is fine.
The disk space is also different since I reflashed... idk.

At this point I am thinking about reverting to stock firmware so I can have an internet connection at home, but that seems to be another challenge... I tried a method found in this forum with no success (in fact I sincerely don't understand how taking the first 8 MiBs of a squashfs image shouldn't result in just garbage, but I tried it anyway), maybe I will open another topic about it...