I’ve read your last post a few times and the configuration is still unclear to me.
1 You have a wired connection on eth0 with a static IP address on your “wired home network”. What does eth0 connect to? A home router, possibly with its own WAN interface?
2 You have a “mobile broadband” router connected on a USB port that acts as a DHCP server and DNS resolver. So does your Pi have two IP addresses when you run ifconfig or do you only see eth0 and lo (and possibly wlan0)? BTW is the mobile broadband router on of those USB dongles?
I’ve had mixed success getting 3G and 4G modems to run on routers, though never had a reason to try on a Pi. For me it was very much a hit and miss affair, with things like Huawei HiLink causing real problems (though great when hooked up to a Windows laptop). Documentation for these devices is generally somewhere between poor and non-existent. Russian-language forums can sometimes provide answers but it’s not easy for non-Russian speakers, as I’m sure you can imagine.