transportation from the airport to guesthouse

Take the airport bus to Reykjavík, and change there to local bus 1 towards Hlemmur, alight at stop Rádhúsið.

Take the Flybus.

A large bus takes you to the Reykjavik Bus Depot where you transfer to smaller buses that drop you at a stop near your hotel. Your Stop is #1, City Hall.

Each smaller bus has a different color sign. They will tell you which one when you board at the airport.

Another vote for Flybus.
There used to also be GrayLine, with a similar service.
Not sure if it’s still operating.
The smaller bus may even drop you right at your hotel, if it’s on the driver’s route.
You can book in advance, or buy a ticket at the airport when you arrive.

First question is how many of you are traveling?

If it just one or two people, Flybus is good. If 3 or more, I would opt for a taxi. The price will be just a tad bit more than Flybus. Flybus takes people on a large coach to the central bus terminal, where you are moved to a smaller bus that will drop you at a bus stop in Reykjavik. So not necessarily right at your hotel. You may have to walk a couple blocks. With luggage, that isn't fun. With a taxi, they will drop you right at your door. So it just depends on what you want. We have done both. I don't mind Flybus when the bus stop is with a block of the hotel, but the transfer can be a bit chaotic. On one trip, heading back to KEF my husband said he just was not in the mood for that, so we had the hotel call us a taxi and that was much nicer way to go. That was Feb 2022 and it ran us $120 US dollars.

I would be a bit concerned about a guesthouse that doesn't respond to questions. Did you book directly with them?

Thanks for all the replies.