Nah it's likely just basic WiFi interference, the OP is just better off grabbing a set of MoCA adapters as i'm betting both upstairs and downstairs have Coax going to them, and they will not have to run anything "new"
In 2023, there's absolutely no reason to seperate the WiFi networks anymore as the routers now handle band steering just fine, especially if the OP got the Nokia gateway.
I'd personally just grab a pair of MoCA adapters from Amazon, and an AirTies from eBay and install the mesh router upstairs with the MoCA adapter so the OP has a close AP and full 300Mbps ethernet for the PC to connect too.
I mean just grab TP-Link's WiFi 7 router.
https://www.tp-link.com/us/home-networking/wifi-router/archer-be900/
4x2.5Gbps LAN port, 2x10Gbps WAN/LAN ports, SFP, and another 1Gbps LAN port.
For a serious, future proof upgrade... i'd not look at anything else right now.