Hello!
I don’k know with Flint but I did without problems on AR750S.
In Luci.
Network / Switch
Make a new VLan for example 3 or 56 what you want and assign a LAN port and make tagged CPU and this new VLan only. Make Off on the new VLan in other places. Only CPU and Vlan 3 or 56 tagged. Save, Apply.
Network / Interfaces
Create a new interface. Name wanb for example. Make it DHCP client. Save Apply.
Edit new interface
1.Physical Settings, select on the list Ethernet Switch eth0.3 or eth0.56 (examples above).
2.Advanced settings set Metric to 30.
3.Assign Firewall settings to wan zone. Save Apply
Install luci-app-mwan3 package. Logout and login again in Luci.
Network / Load Balancing should appear in menu.
1.Interfaces. Add. Name it wanb again if you want. Check Enabled - Online. Save
2.Members. Add. Name it wanb_only if you want and THAT’s IMPORTANT, put same Metric and Weight as wan. Save
3.Policies. EDIT the one already made and add inside the options the new wanb. Save. Apply.
Connect the cable on the port you tell is wanb, may be port 1-4 in the Flint. Look in Luci/Intefaces if wanb have and assigned IP given by and from the 5G cell router. If ok, connect internet both devices.
In Luci again go to Status / Load Balancing and look the colours if green both wan and wanb that’s ok, also look in Details what proportion they have. If well done it must be 50% wan and 50% wanb.
I don’t know if Flint has same things in Luci than AR750S.
Also the same thing is possible with wwan if you connect to your 5G cell router by wifi and you don’t have to make new interface nor vlan, only Load Balancing configuration in wwan.
Bye