ok. i know mellanox as brand. its now called nvidia.
the network cards are generally good, some of the best even.
the model you are having is just ancient and EOL in terms of driver support. that means in windows anything after probably windows 7(10 at best, with some hack maybe) wont be able to use it. the last official drivers are for windows server 2008 r2 which equals windows 7.
linux driver support is generally much longer lived, so the chance of your card working on linux are generally higher, but even there development for the drivers most likely stopped a long time ago and its possible that the generic mellanox drivers dont support this card anymore.
i had a connect-x2 card which i used on windows 7 for a few years but ultimately discarded it when windows 10 came out as i couldnt make the old drivers work anymore.
googling around a bit it seems like the MT26448 cards are hit and miss depending on what kernel is used and what firmware is on the card.
specific to proxmox you can find people having issues after using pve 6.x or newer while the card worked fine on older versions (and thus older kernels).
you will have to try if you can still make it work, but my recommendation would be a newer, more modern card.
for example there are rj45 and sfp+ versions of the brand new realtek rtl8127 cards available for relatively little money(40-80€ new).
they are available as pcie4.0x1 or pcie3.0x4 cards (and even m.2) and are brand new.
chances are high that they will be supported by linux for a considerable amount of time to come.
otherwise second hand enterprise-nics are available as well under 100€, namely the connect-x4, intel x520/x540/x550/x710.
i have seen connect-x4 cards as cheap as 40€ on aliexpress.
those should all still be reasonably supported on current kernels.
