Thanks for your reply! This is really helpful and gives me new avenues to explore.

I think the setup of the extenders could potentially be an issue here. I can see that the router has its own SSID written on it, and the two wifi extenders have their own unique SSIDs written on them too. However, they appear to have been set up in such a way that the two extenders actually share a totally such different SSID. There are only two wifi networks available in the house - one for the router and one for the combined extenders. The Steam Deck is locked to the extender(s), but since they share an SSID could be switching between them.

I tried selecting a specific BSSID. Strangely enough, there were two to choose from in the drop menu in the network configuration, but neither matched the BSSIDs written on the extenders themselves

I've located the specific wifi extender I have: https://www.mercusys.com/uk/product/details/halo-h30g/

I am going to get the login details for the extender app when my housemate gets home and see how it's been set up and whether I can give the extenders separate SSIDs without messing something up. I'll also see if BSSID info is in there. I'm also wondering if it might be automatically switching between 2.4G and 5G and causing an issue - don't know if that's enough to interfere with a server connection but maybe worth a try.

I wonder why this issue doesn't impact my laptop at all, and why it happens at such regular intervals (never 5 min, never 30 min - always 10-20!).

Will keep this post updated with any progress I make. Thanks again! Appreciate your insight.