How to share internet between separate networks?

[LeahaTaul]

In a temporary living arrangement, a friend has internet access through an ethernet cable from an existing router. To create his own network, he wishes to use his own Nighthawk router and connect it to the existing router to provide internet to his devices, which includes a NAS, docker server, and others. However, he wants to maintain separation between both networks to prevent devices from each network from seeing each other. The question is omegle owespeedtest whether it is possible to achieve this without administrative access to the existing router that is providing the internet.