OP has the use-default-gateway=yes set... otherwise the /ip/route wouldn't be there.
It's an interface route, and modem knows the destination is the other end of the LTE "session". So there is no IPv4 gateway.
The "actual" routing table is in /routing/route/print (/ip/route is more like a filtered+ordered view of that info). If you look at an LTE default route there, you'll see the routing table knows it "modem" - so gateway is numberless/"other-end of the LTE link", see belongs-to="modem" for an LTE route:
/routing/route/print detail
Flags: X - disabled, F - filtered, U - unreachable, A - active;
c - connect, s - static, r - rip, b - bgp, o - ospf, i - isis, d - dhcp, v - vpn, m - modem, a - ldp-address, l - ldp-mapping, g - slaac>
H - hw-offloaded; + - ecmp, B - blackhole
Am afi=ip4 contribution=active dst-address=0.0.0.0/0 routing-table=main gateway=lte1 immediate-gw=lte1 distance=2 scope=30
target-scope=10 belongs-to="modem"
debug.fwp-ptr=0x20282120
But you can use "lte1" as the gateway in any additional routes+tables, as if it had a GW.