Asus Router w/ Merlin FW

[Romanze]

Hello I have an Asus RT-1750 router wired to my computer. My ISP is AT&T 1GBps. I have Merlin FW version 380.70. I used to get~900MBps up and down. Lately I can only get max 40MBps. I noticed when I connect to my VPN then run a speed test it jumps up to about ~600 MBps up and down. I cannot figure out why all of the sudden my speeds are slow, but then speed up on a VPN of all things. I tried restarting modem, router, pc. I have tried Windows 10 and Linux. I dont know what changed and why I cant get those speeds anymore. Furthermore, to every ISP's advice I have tested directly connected to the modem and it is around ~900 MBps up and down.