Hidden router settings

It sounds like you are looking for what I would consider to be Quality of Service (QoS) related settings. In my experience QoS usually isn't an option in simpler / less capable / lower end routers, including many multi-function modem router combination devices.

IMHO there is a very real chance that there is no option to do what I think you're wanting to do in the device that you're using.

Assuming your current router doesn't have the required QoS settings, and assuming your current router can't run a 3rd party firmware that does includes QoS, you could potentially daisy-chain a second router which offers QoS. Before doing so, think about WiFi access and port forwarding, for starters.

Maker and model number of the router? Firmware version? QoS is usually by service type. For example, you can reduce delays to real time apps (VoIP, gaming, video) at the expense of delaying apps that don't require instant response (web, email, online shopping, etc). That doesn't sound like what you want.

More likely, you need a bandwidth management or traffic shaper feature. These are usually found only in high end routers or in external boxes (usually computers). For example:

Note that consumer grade routers usually do not have the CPU horsepower to do traffic limiting and will tend to run slow when such features are enabled. This is not a problem when you're on a slow DSL internet connection, but can be a real bottleneck if you're on a fast fiber or cable internet.