These rules & route are necessary to properly send packets on oaitun_ue1
interface:

1. This forces the packets coming from a subnet different than the UE's
   subnet to go back through oaitun_ue1 rather than via what the default
   route defined on the system (e.g., important if ping from the
   internet arrives).

2. On machines setting net.ipv4.conf.oaitun_ue1.rp_filter=1 (e.g.
   RHEL), this prevents that source filtering for packets coming back is
   applied and those packets be dropped. By default, many hosts have
   rp_filter=2, so no strict source filtering is applied, and it would
   work; on others, this rule prevents source filter dropping.