TCP/IP networking is TCP/IP networking. If you plug the right IPs in the right places, it shouldn’t matter one bit what OS you’re using to connect, so long as you have a working NIC with drivers and it supports TCP/IP correctly.
Our old ISP “didn’t support Macs”...by which they meant their installer cds didn’t. If you set up the connection manually, it worked just fine.
Windows 98 might not have L2TP/IPSec drivers available, for example. Or maybe ISP is using IPv6, which has only the barest sliver of support on Windows 98.