How do I enable 5G?

Hmm thats hard to say. Even if it is a 5G compatable device, koodo has a strict whitelist of devices that are actually compatable fully with their network. This mainly includes main phone brands such as Apple, Samsung, Google etc.

Since changing your plan, have you restarted the device or reset your network settings? Give this a try and see if it prompts 5G to appear in the preferred network. If that doesnt work, double check that your APN settings are correct by following the article below.

If none of those work, its likely because the device is not apart of their whitelist. That, and I noticed that model is missing the n78 5G network band which is one of the main bands koodo uses, so this could also be the cause.

https://www.koodomobile.com/en/help/setting-data-your-non-koodo-phone?srsltid=AfmBOoqsm4i4JSki2Js_Mz7B9TqqtUAvUjvoL4PTAzUjiM_ybYlz6Fkr

I know Koodo has a whitelist for devices for VoLTE / wifi calling. 

I have not heard of Koodo having a white list for connecting to a 5G network.

Even if you have a 4G plan, you should be able to connect to the 5G network if your device is compatible.  A 5G plan just means your network speed is not throttled to 4G speed so changing plans will not change this.

From my searching, it doesnt look like OnePlus 8 (Model IN2015) is compartible for 5G on Koodo’s network.  It is only compatible for 5G on the T-Mobile network

Well actually no, it does have the n78 band. Or at least that’s what https://www.oneplus.com/global/8/specs says.

Have you tried this method? https://community.oneplus.com/thread/1345340

Yup, that is my mistake for trusting a quick google search. After spending more than a few mins on it  I was able to find the specs on One Plus's website that also said it had the network band.

From further searching though I agree with Dennis, it doesnt seem to be compatable with the Koodo 5G network straight away. I did see that people found work arounds as provided by MilkyWay so maybe that is worth a shot.

To add on to Oliver, your phone model is rather old, which is you may need to resort to this method. Based on what I know, later models of the OnePlus do not seem to have an issue getting access to the 5G network.