Long post incoming:
My brother, and I have the same issue with Suddenlink Gigabit Internet, and both of us live about 20 minuets away from each other. Suddenlinks routing is garbage, and when the game first came out I was getting 25-30 ping on every west coast server I joined. Then all of a sudden my ping started hitting between 75-150 on every game, and if I disconnected mid game several times it would eventually put me back to 25-30 again. Though that became rarer and rare.
Needless to say that wasn't really feasible when your trying to play ranked, and even then it was no guarantee that you'd get normal ping with each attempt. So I reached out to Riot support thinking it was an issue on there end. Since any other game that I played had normal ping and only Valorant was giving me this issue. Their support was amazing with helping me figure out where the problem was occurring. Had me running tracer tests, and several other network diagnostics to their west coast host IP address, and after a day or two of back, and forth they determined that it was my ISP that was causing the issue. I also discovered that wasn't just Valorant either, but my normal web browsing was having the same problems as well. Though given that it's only normal browsing that is routing me that way. Though it's not as noticeable as playing a fps game where ping is the difference between making kills in higher tier game play. Which with Valorant seems to be more than any other game I've played.
At first I didn't believe the person from support thinking why would they want to take accountability, but after studying the issue, I learned that it was something that ISP's do with their routing to curb network congestion, and also to save money with their connections that they pay to route through at each server center. Every now and then the game will connect normally, and I don't have issues, but more often it's the high ping. Which made the game feel unplayable/effect my performance heavily.
So I went on the hunt for a routing service, or VPN/GPN that would route me through the best path. You name a VPN/GPN service, and I either used their free trial, or bought 1 month of their service to try to fix it. I had a few hopeful ones that would bring my ping down to 30-50 again, but the stability of the servers that they connected were horrible. Lag spikes, packet loss, and disconnects plagued me with each one. I won't even mention all of them, but every big name service was tried.
The last service that I ended up using was a weird looking application called MudFish, and when I first installed it to see it was operated through your browser I sorta ignored it. Also due to the fact that it didn't have Valorant support when I first installed it. Continuing to struggle to get a decent connection I started going back to the services that sorta worked to give them a shot again. I then read either a random hidden reddit post, or forum post about how MudFish actually worked for someone with the same issue. So I reinstalled it, and looked it over again. It now had Valorant support, and when I seen how cheap the service was I was ($0.83 a month) thinking it would be garbage like the rest. Then I seen how many server nodes it had available, and my mouth about hit the floor. Not only that, but the service was a true GPN with only the routing to the game itself effected by the applications connection. All other traffic was left alone.
I found about 30 server nodes that I could connect to in the area I needed to be in. The first time I connected in game I had 75 ping, and I felt like it was not gonna work for me. Then after I rebooted the game, I then disconnected the service, and reconnected to give it another shot. When I joined the range I had 25-29 ping. I then joined a few spike rushes, and the ping was stable the entire time. While I had a few alerts in the MudFish browser that I was having a packet loss on occasion. This wasn't translating over into the game, and the ping was super stable with no lag spikes, no packet loss, and no disconnects. Needless to say the game felt better than ever. I am not sure if anyone has gone into the range with 80 ping, and did some head shots. Then gone in with 25 ping right after, but the registry of your bullets connecting is like night and day snappier with lower ping. Bullets at 75-150 ping are super delayed at hitting the target.
After a week of playing with stable ping, and realizing I had finally found the silver bullet that conquered my issue that I was having (at least until my ISP somehow changes on its end). I then noticed the service also told me exactly which nodes the Valorant server was routing me through as well. Giving me the option to pick my connection to the exact server node that was going to give me the shortest path. So I am now connected to a server node in Fremont, and my games bullet registry feels great. Allowing me to place my way to plat/diamond again. I was having a hard time staying in gold when I was playing with 75-150 ping.
I just wish this information had found me sooner, and after setting my brother up with MudFish this last week, he's amazed with it as well. So I did a google search with "MudFish & Valorant" which brought me to here, and I wanted to share this experience with you so that you, or another person who's having this same problem get help sooner. I am not endorsed by MudFish or am I a shill for them. Though I am a believer of giving people the information that they're looking for in order to help them out with something that can be fixed relatively easily.
Anyways I hope this long ass post about my story, and the information helps you, or if your reading this having the same issue as me and OP had. Then download this program, and give it a shot before you even try, and use any other. It hands down beats them all out by a mile.