Here are just a few. There are more if you dig deeper:

https://cybernews.com/security/walmart-exclusive-routers-oth...

https://arstechnica.com/information-technology/2014/04/easte...

https://www.zdnet.com/article/multiple-backdoors-and-vulnera...

Even Cisco was doing it: https://www.zdnet.com/article/cisco-removed-its-seventh-back...

And the NSA was known to be intercepting router shipments to international customers, injecting their backdoors, then re-shipping the modified hardware:

https://www.infoworld.com/article/2608141/snowden--the-nsa-p... (this is documented all of the place; infoworld may not be the best source but it is just one)

For every example that is exposed, it is safe to assume there are others that have not been found.