If I have understood you right, then:
Option 1: main router → wifi → Asus Router (OpenWrt) → cable → Client Laptop = lots of jitter
Option 2: main router → wifi → Client Laptop = less jitter
This experience could be explained by:
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Option 2 makes use of your Laptop's network card and antennaes. It could be that this network card is simply more performant (for whatever reason) than your Asus RT-AC51U. It would be interesting to know, the name of your laptop's network card and what operation system do you use + drivers / firmware version.
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Option 1 and Option 2 are conducted at different locations. Location is important when it comes to wifi performance
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Jitter could be introduced on the path from your main router to the fast.com instance. I would suggest to focus on trying to find the culprit within your own local network first, before you focus on something you have no control over. You could do this with one of the various measurement tools available, for example Iperf2 or Flent. I have collected some at Best practices and tools for measuring wifi performance.
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Your cable is broken
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It could be because of a bug
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It could be because of the way you configured your device.
To let us know about your configuration, first, access your device (e.g. via SSH (See how: https://openwrt.org/docs/guide-quick-start/sshadministration)) and start the command line (CLI).
Then please copy the output of the following commands and post it here using the "Preformatted text </> " button.
Remember to redact passwords, MAC addresses and any public IP addresses you may have:
ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall