I just tried it on my Roadwarrior Carambola2 and worked like a charm.
Installed both TRM and Luci-app. I did the changes in Luci, but most things were left to defaults.
/etc/config/network
config interface 'trm_wwan'
option proto 'dhcp'
/etc/config/firewall
config zone
option name 'wan'
option network 'lte wan wwan trm_wwan'
/etc/config/wireless
config wifi-device 'radio0'
option type 'mac80211'
option hwmode '11g'
option path 'platform/ar933x_wmac'
option disabled '0'
option htmode 'HT40'
option country 'CZ'
option legacy_rates '0'
option distance '30'
option channel '11'
config wifi-iface
option ssid 'trendy-hotspot'
option encryption 'psk2'
option device 'radio0'
option key 'SomeKey'
option mode 'ap'
option network 'lan'
option wpa_disable_eapol_key_retries '1'
config wifi-iface
option network 'trm_wwan'
option device 'radio0'
option mode 'sta'
option ssid 'trendy-guest'
option encryption 'none'
option disabled '0'
travelmate status
::: travelmate runtime information
+ travelmate_status : connected (net ok/45)
+ travelmate_version : 1.2.2
+ station_id : radio0/trendy-guest/-
+ station_interface : trm_wwan
+ faulty_stations :
+ last_rundate : 11.10.2019 00:07:25
+ system : 8devices Carambola2 board, OpenWrt 18.06.2 r7676-cddd7b4c77
In my debugs I can see clearly the message that it connected:
Thu Oct 10 23:58:12 2019 user.info travelmate-1.2.2[13007]: connected to uplink 'radio0/trendy-guest/-' (8devices Carambola2 board, OpenWrt 18.06.2 r7676-cddd7b4c77)
which I don't see in your logs. However I think that in f_jsnup lines it shows some connection, which is lost later. What I don't have at all is the f_net, most likely to detect captive portals, which results to nok so maybe something is wrong there.
I don't have any ideas, so unless someone else has, maybe you could ask user dibdot.