
NanoPi R5S router SBC/gateway with 2x 2.5GbE, 1x Gigabit …
Your (initial) problem is: not remembering your R4S's IP address. And this problem is solved since 1.5 decades by DNS-SD/mDNS (link local service propagation) . Even if you're on a 10/8 network and your R4S has 198.18.1/24 (ping won't work) as long as both devices are connected 'link local' the R4S will happily announce its 198.18.1.x ...