Two routers via one cable from the provider (Qotom-->SNR-->Provider)?

Question

Computer networks

At the moment, there is a router SNR-CPE-MD1.1, a PC is connected to it via a cable, the rest of the clients are using wi-fi. One of these days, I get Qotom Q150P-S08, pfsense + openvpn will be spinning on it, I will connect a PC to it. And so I decided to think in advance, if I connect Qotom to SNR via LAN, will it affect the performance of SNR due to the use of OpenVPN on Qotom or is it just transit traffic that will not affect the performance of the "main" router?
How can I properly connect SNR and Qotom through one cable from the provider?