That might be an issue given that the injector would need to be on the cable directly connected between the ASUS router WAN port and the 4G modem.
The ASUS is probably not expecting to see PoE on its WAN port, which may or may not be a problem.
That�s not a problem. Use a short cable from ASUS to PoE injector. Then long cable from injector to your 4G modem. ASUS has no knowledge of injector..
plusnet 80/20 (2/jun/14) at 470m; high sync history: 64/9 (Sep/17), 54/6 (Jan/19), 51/6 (Mar/19), 47/6 (Aug/19)
20 years of broadband from 1999's ntl:cable modem trial - Live BQM