Edit to clarify: most houses I’ve lived in, the internal doors could easily fit an Ethernet cable under them when closed. It’s not pretty but if it’s mostly routed along walls behind furniture you’ll likely not notice it that much.
100mbit is not “typical” for new, non bargain-basement equipment in 2020.
If they aren’t connecting at gigabit reliably one of the three things (router/switch, computer, Ethernet cable) either not rated for gigabit or is faulty.
I had some similar symptoms with an isp provided Huawei fttp ont+router. I ended up plugging the computer (along with a network a3 printer) into a little desktop gigabit switch and it’s all been peachy since then.
The moral of the story is that isp provided devices are usually worse than bargain basement because you aren’t even getting the savings.
If possible I’d either replace it, or if you’re on eg docsis (aka cable) and can’t find a reliable one aftermarket, put their device in bridge mode, and run a good quality router of your own.