In my opinion, the optimal way is to look at @Chupaka’s post on scriptless failover based on recursive next-hop search, create a VLAN on the cable between the wired router and the wAP AC with a dedicated connection subnet in it, and configure that VLAN/subnet wAP AC as a secondary WAN subnet for the wired router. To reduce LTE data plan consumption, you may not verify the transparency of the LTE link, as it is secondary anyway so if it doesn’t work, there is no backup left.