You should also be able to use your ISP (Fibre) supplied Router and allow all you clients to access the internet normally and then add your DSL-AC82U and use this as a secondary router and enable NordVPN on your DSL-AX82U and then only clients that connect to the DSL-AX82U will use the VPN only.
Note: some ISP supplied routers do not support VPN Passthrough, so this could be an issue due to double NAT etc. but worth a try anyway.
Simple diagram below which may help and lots of alternatives.

In a completely sane world, madness is the only freedom (J.G.Ballard).
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