Hi @jaime
Welcome to the IPFire community.
Your problem sounds similar to a bug that was present a couple of years ago. It took a while to find a solution but was eventually fixed in Core Update 165. However if it is the same then you need to do some manual intervention, using setup, on the red interface settings.
Some ISP’s have cable modems which return an unusually small MTU of only 576 bytes which will cause that IPFire will fragment every packet larger than this before it can be sent out on the RED interface.
See the first item in the Misc section in this release announcement for CU165
https://blog.ipfire.org/post/ipfire-2-27-core-update-165-released#misc
The original thread on this bug topic was
https://community.ipfire.org/t/google-meet-stopped-working-after-core-update-152-153/4272