As Vexira said, you should try to DMZ the Asus router in the 4G router. I have the same situation, piece of s***t provided by the ISP, a Huawei B315-22s and has no bridge mode at all. My setup's:
4G > Huawei (192.168.8.1) > WAN port > RT-AC68U (192.168.10.1) > switch & APs
On Huawei went to...