Since I see it would be possible to load openwrt on mercusys MB130-4G, with tftp ? could it be compatible with MB135-4G lte modem ?
Usually not directly compatible, no. Typically, a different model indicates some hardware differences in the design (even if visually similar/identical, it may have different chips or other changes under the hood). There are some situations where the exact same device is marketed under different model names, but this is not terribly common.
As such, it is necessary to investigate the 'new' device to figure out what chips it contains as well as key information about the rest of the system (including the pins used for I/O, ports, etc.).
Since you have the device in hand, you can start that process:
I ordered one (seems relatively cheap) hoping it might be similar to this MB130-4G https://git.openwrt.org/openwrt/openwrt/commit/?id=b8eba7514643cc6b64d93e3fa7048d6721efbc17
in case anyone might have any clue could MB135-4G be similar to MB130-4G which has openwrt support ?
I’ll try if it has any telnet/ssh console, maybe open it if not ?
they’d have released the GPL tarball here
diffing against MB130 gpl tar seems possibly compatible ?
and diff with MB130 openwrt version ? the goal would be to arrive at “target” and .dts (device tree source) files modifications ?