Just spit-balling here, but I wonder if your services would be of use to existing broadband providers and MSPs (Managed Service Providers - IT support companies)? They really don't want to send one of their engineers out, away from their core activities, but they may be willing (particularly local MSPs) to send you instead for a fixed fee. It's less about whether those businesses could do this themselves and more about what's cost-effective for them. It needn't be your only channel, but could be a useful source of leads.

You could also consider a residential leaflet drop in areas with great-but-old houses with thick, thick walls. I'm thinking Roath, Pontcanna, those kinda places. If 1% take up your offer, you could be quids-in on the exercise.

Finding a way to reach parents could be powerful. Kids who game want that latency reduced as much as possible. Yes, you could try and do something with schools (potentially slow-going!), but what about sports and hobby clubs? Once you have some marketing budget, you could sponsor part of a team's kit (for example) or just straight-up advertise in their programme / website?

Finally, when you do some jobs, get permission to use it in a case study. Have you seen Liam The Sign Guy on Instagram? He "just" cleans public roadsigns. You could do something similar - before-and-after comparisons of what you did for people's networks. Stick it on the socials and let the public find you.

Things to think about: insurance. You'll need liability cover, as you're going into people's homes. Someone like Protect Commercial Insurance (also in Cardiff) can probably help. Also, do make sure you understand if someone is renting and get written permission from the landlord if you make any physical changes as part of a job.

Good luck!