UTI's are fairly common in women who are sexually active, but how were they treated in the past before modern antibiotics?
Here is an article available from PubMed which addresses this:
The article is behind a paywall, but if you read the abstract it says that treatment of UTI before antibiotics was palliative - that is, directed towards relieving the symptoms rather than effecting a cure. Looking at the list of treatments, it's not too clear that they would have relieved the symptoms either. They range from useless (bleeding, herbal enemas and douches) to possibly useful (plasters to relieve pain, narcotics ditto, bed rest in general to help the body heal faster). There are some herbs with anti-inflammatory properties which could have helped relieve the pain, but they are tricky and nonstandardized, and how well they performed would depend greatly on the skill of the doctor. They would not shorten or cure the disease, however.
Cranberries are a new world plant and were not available in Europe before the Columbian exchange. There is some literature saying that cranberry juice may help prevent (but not cure) UTI, but more recent studies show no effect.
The one thing that could help cure it faster - drinking lots of water - is not mentioned.
The vast majority of cases of UTI are minor and self-correcting, meaning it goes away in a few days even if untreated. In serious cases where it led to chronic cystitis with abscesses or strictures of the urethra, surgery was available. Catheters are mentioned in the Sushruta Samhita from India (600 BC) and bronze catheters have been found dating from the time of Charaka in India and Hippocrates in Greece. These could have been used to relieve strictures and drain the bladder. Surgery was also available for bladder stones, which are a complication of chronic bladder infection.
You may be interested in this past question: How did women treat UTIs in the past?
I'll reproduce my own contribution here, with the preface that pre-modern medicine generally did not function on a "pathogen-disease" model but more of a "symptoms-disease" model, to put it crudely. This is simply a reminder that while we might classify something as a bacterial infection of the urethra might instead be understood primarily through it's symptoms in the past, including dysuria and groin/torso pain. Anyway, here are some treatments from the late Aztec/early Colonial period:
There's a couple sections in the Badianus Manuscript, compiled in 1552 and reflecting Aztec medico-herbal traditions (with some Euro introgression), which could fit the prescription. They deal generally with "pains of the groin" and "difficulty in urination," which are of course classic UTI symptoms.
The former symptom calls for grinding a type of pine with two other unidentified plants, tlanen popoloua and "those that grow in a garden once burned over," along with sapote, brambles, and rushes together with a gizzard stone from a swallow. This should all be ground together with swallow and mouse blood and applied topically.
With the latter symptom a drink is prepared from flowers from leeks, an undentified Bourreria sp. (tlaco itzquixochitl), and magnolia blossoms. Also to be added are an unidentified root (mamaxtla), white and red earth, and ground up stone called an eztetl. This should be drunk and then oysters are to be placed on the stomach.
For urinary retention, i.e., when "the flow of urine is shut off," a different drink is prescribed. This one includes mamaxtla, the roots of Dorstenia contrajerva, rushes, and magnolia blossoms. This should all be ground together in acrid water with the "tail of a sucking puppy" and macerated chia seeds. An enema of sorrel root crushed in hot water could also be prescribed.
Although if all "this medicine avails nothing" then it would be necessary (with men) to:
take the pith of an extremely slender palm, covered with thin cotton and smeared with honey and the crushed root of the herb huihuiztmallotic (unk.) , and this is cautiously inserted into the virile member.
And FYI, uncomplicated UTIs can and do resolve without antibiotics in healthy individuals. It's just that they do have a small chance to progress to a more systemic infection leading to things like kidney failure and death.