Yes, I can understand your frustration, just need to clary exactly what you are trying to achieve.
Is the DSL-AC88U going to connect to another router/modem or just modem.
Do you plan to use main router/modem wifi, dhcp etc
FYI – Having two routers creates two networks with two DHCP servers and two NATs, this causes an issue when port forwarding and UPnP need to be employed on devices (eg computers) connected to the 2nd router.
if so, then:
modem/router will be called – primary router
wireless router will be called secondary router
In a completely sane world, madness is the only freedom (J.G.Ballard).