Network issue after new update

Which new update are you referring to? 

• iOS Update? Which Version? Update your iPhone, iPad, or iPod touch - Apple Support
• Carrier Update? Contact Carrier. Update your carrier settings on your iPhone or iPad - Apple Support
• App Update? Which App? Contact App Developer? How to contact an app developer - Apple Support

With the limited information that you have provided, I had to guess the problems faced by you. Some of it can be understood by reading below

Learn and set Recommended settings for Wi-Fi routers and access points ...

2.4GHz radio mode

This setting controls which versions of the 802.11n/ac standard the network uses for wireless communication on the 2.4GHz band.

Set to: Auto or 802.11n/ac

Routers that support 802.11 should be configured for 802.11n/ac for maximum speed and compatibility. Different Wi-Fi routers support different radio modes, so the setting varies depending on the router. In general, enable support for all modes. Devices can then automatically select the fastest commonly supported mode to communicate. Choosing a subset of the available modes prevents some devices from connecting. For example, 802.11ac devices can't connect to a Wi-Fi router in 802.11n-only mode. Also, choosing a subset of the available modes might cause interference with nearby legacy networks, and nearby legacy devices might interfere with your network.

5GHz radio mode

This setting controls which versions of the 802.11a/b/g/n standard the network uses for wireless communication on the 5GHz band. Newer standards support faster transfer rates, and older standards provide compatibility with older devices and additional range.

Set to: Auto or 802.11n/ac

Routers that support 802.11n should be configured for 802.11n/ac mode for maximum speed and compatibility. Different Wi-Fi routers support different radio modes, so the setting varies depending on the router. In general, enable support for all modes. Devices can then automatically select the fastest commonly supported mode to communicate. Choosing a subset of the available modes prevents older devices from connecting. For example, 802.11ac devices can't connect to a Wi-Fi router in 802.11n-only mode. Also, choosing a subset of the available modes might cause interference with nearby legacy networks, and nearby legacy devices might interfere with your network.

Channel

This setting controls which channel your Wi-Fi router uses to communicate.

Set to: Auto

For best performance, choose "Auto" mode and let the Wi-Fi router select the best channel. If this mode isn't supported by your Wi-Fi router, choose a channel that's free from other Wi-Fi routers and other sources of interference. Read about possible sources of interference.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

Hi

2.4 GHz is more penetrative

You can sit further away from router

5 GHz is more suitable for more data at once, like big files or large images.

Well, in general.

Anyhow, I don't think it's related to any ios or MacOS update.

Just restart all devices, including the router.

See if that helps

2.4 GHz connections are about 20 times slower than 5 GHz. It has nothing to do with your phone or your router. It is the way Wi-Fi is designed; the 2.4 GHz band is a much older standard. So you will have to use 5 GHz if the ping on 2.4 GHz is not within the standard of the game you are playing.