Thank you for everybodies help, I got it working!! 
Not exactly sure what fixed it. I removed the earlier mentioned QMI generated interface and followed instructions for ModemManager https://openwrt.org/docs/guide-user/network/wan/wwan/modemmanager
So I ran this:
opkg update && opkg install kmod-usb-serial kmod-usb-net kmod-usb-serial-wwan kmod-usb-serial-option kmod-usb-net-qmi-wwan kmod-usb-net-cdc-mbim luci-proto-modemmanager
And then followed the screenshot for Luci ModemManager after a reboot and added the newly created BROADBAND interface to wan firewall rule.
As mentioned before, earlier tries with ModemManager failed, possible I installed some missing package. Probably upgrading to latest OpenWRT version and powering off the interface before inserting SIM seems to have helped as well (since at one point those cli commands related to LTE gave no output initially).
And in case it helps anybody, here's the current kernel modules:
root@openwrt:~# lsmod
ath 18944 4 ath9k,ath9k_common,ath9k_hw,ath10k_core
ath10k_core 445568 1 ath10k_pci
ath10k_pci 36304 0
ath9k 103024 0
ath9k_common 11904 1 ath9k
ath9k_hw 347840 2 ath9k,ath9k_common
cdc_ether 5120 1 cdc_ncm
cdc_mbim 4608 0
cdc_ncm 15616 1 cdc_mbim
cdc_wdm 8512 3 cdc_mbim,qmi_wwan
cfg80211 285648 5 ath9k,ath9k_common,ath10k_core,ath,mac80211
cmac 2304 1
compat 992 4 ath9k,ath9k_common,mac80211,cfg80211,[permanent]
crc_ccitt 1664 1 ppp_async
crc32c_generic 1376 1
drbg 17536 0
ehci_fsl 3456 0
ehci_hcd 38128 2 ehci_platform,ehci_fsl
ehci_platform 4992 0
fsl_mph_dr_of 3632 0
gpio_button_hotplug 6592 0
hmac 2496 0
hwmon 8384 1 ath10k_core
jitterentropy_rng 7136 0
libcrc32c 592 1 nf_tables
libsha256 8640 1 sha256_generic
mac80211 523424 2 ath9k,ath10k_core
mii 5152 1 usbnet
nf_conntrack 69104 7 nft_redir,nft_nat,nft_masq,nft_flow_offload,nft_ct,nf_nat,nf_flow_table
nf_defrag_ipv4 1216 1 nf_conntrack
nf_defrag_ipv6 5584 1 nf_conntrack
nf_flow_table 22224 4 nf_flow_table_ipv6,nf_flow_table_ipv4,nf_flow_table_inet,nft_flow_offload
nf_flow_table_inet 576 0
nf_flow_table_ipv4 448 0
nf_flow_table_ipv6 448 0
nf_log_common 2912 2 nf_log_ipv6,nf_log_ipv4
nf_log_ipv4 3584 0
nf_log_ipv6 3904 0
nf_nat 22032 4 nft_redir,nft_nat,nft_masq,nft_chain_nat
nf_reject_ipv4 2560 2 nft_reject_ipv4,nft_reject_inet
nf_reject_ipv6 2880 2 nft_reject_ipv6,nft_reject_inet
nf_tables 148464256 nft_fib_inet,nf_flow_table_ipv6,nf_flow_table_ipv4,nf_flow_table_inet,nft_reject_ipv6,nft_reject_ipv4,nft_reject_inet,nft_reject,nft_redir,nft_quota,nft_objref,nft_numgen,nft_nat,nft_masq,nft_log,nft_limit,nft_hash,nft_flow_offload,nft_fib_ipv6,nft_fib_ipv4,nft_fib,nft_ct,nft_counter,nft_chain_nat
nfnetlink 5152 1 nf_tables
nft_chain_nat 864 2
nft_counter 1728 24
nft_ct 7616 3
nft_fib 1376 3 nft_fib_inet,nft_fib_ipv6,nft_fib_ipv4
nft_fib_inet 672 0
nft_fib_ipv4 2080 1 nft_fib_inet
nft_fib_ipv6 2784 1 nft_fib_inet
nft_flow_offload 3520 0
nft_hash 2336 0
nft_limit 3456 5
nft_log 1664 0
nft_masq 1792 1
nft_nat 2592 0
nft_numgen 1440 0
nft_objref 1664 0
nft_quota 1952 0
nft_redir 1760 0
nft_reject 1120 3 nft_reject_ipv6,nft_reject_ipv4,nft_reject_inet
nft_reject_inet 1152 2
nft_reject_ipv4 672 0
nft_reject_ipv6 672 0
nls_base 4896 1 usbcore
option 33344 3
ppp_async 7104 0
ppp_generic 25008 3 pppoe,ppp_async,pppox
pppoe 9568 0
pppox 1296 1 pppoe
qmi_wwan 16608 0
seqiv 1600 0
sha256_generic 2400 0
slhc 5152 1 ppp_generic
usb_common 2736 2 ehci_platform,usbcore
usb_wwan 4512 1 option
usbcore 146400 13 option,cdc_mbim,usb_wwan,qmi_wwan,cdc_ncm,cdc_ether,usbserial,usbnet,cdc_wdm,ehci_platform,ehci_fsl,ehci_hcd
usbnet 17232 4 cdc_mbim,qmi_wwan,cdc_ncm,cdc_ether
usbserial 17936 8 option,usb_wwan
Feels unreal, this has been a dream setup, finally I can retire the other router and learn more about this marvelous project. Got the LTE indicator working by activating it in System > LED Configuration.
THANK YOU!