USB LTE modem without MAC address?

I'm connecting a linux machine using a nowadays popular Huawei Brovi E3372-325 LTE USB Stick to the internet. The special requirement is that incoming ssh/ping/NTP/... connections must reach my linux OS. The state is, that using usb_modeswitch -X and the option driver I can bring up the 3 ttyUSB interfaces, and connect successfully using wvdial. But for some reason, ifconfig does not list a HW/MAC address for ppp0 interface, and devices on the same APN network can't ping the my IP address. I don't think the reason is blockage by the ISP, because my other devices are ping-able on the network. Output of ip addr

19: ppp0:  mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 3
    link/ppp 
    inet 10.250.0.112 peer 10.64.64.64/24 scope global ppp0
       valid_lft forever preferred_lft forever

1. If I'm not mistake, I don't use RNDIS right now. Am I right, that in general the popular RNDIS protocol does not suit my use case, because that creates an additional local network, making it trickier to forward incoming connections to the OS? Pinging might work from outside because that's answered by the USB modem itself, but incoming ssh would fail. 2. What might be the reason, that ppp0 does not have a MAC address? How it's even possible? Should I assign one? Is it probably the reason that other devices can't ping it's IP? How to solve this situation?